Simplify unsigned-to-string conversion code and improve performance

Simplify unsigned-to-string conversion code and improve performance

Summary:
This seemingly trivial change has a surprisingly significant performance
impact. It changes the call to append() on a string-like object from using
an iterator pair to using a char* / length pair for 64-bit unsigned-to-string
conversion. This brings it in line with code for signed-to-string conversion,
which had already been using this overload of append.

  =============================================================
  folly/test/ConvBenchmark.cpp            time/iter   time/iter
  =============================================================
  preallocateTestNoFloat                   640.47ns    590.12ns
  preallocateTestFloat                     569.32ns    580.25ns
  preallocateTestInt8                      133.65ns    116.27ns
  preallocateTestInt16                     147.05ns    130.03ns
  preallocateTestInt32                     169.98ns    156.24ns
  preallocateTestInt64                     228.31ns    210.66ns
  preallocateTestInt128                      4.53us      4.56us
  preallocateTestNoFloatWithInt128           4.32us      4.27us
  -------------------------------------------------------------
  u64ToStringFollyMeasure(1)                17.32ns     15.49ns
  i64ToStringFollyMeasurePos(1)             15.80ns     15.80ns
  i64ToStringFollyMeasureNeg(1)             21.91ns     21.91ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(2)                18.23ns     16.10ns
  i64ToStringFollyMeasurePos(2)             16.71ns     16.71ns
  i64ToStringFollyMeasureNeg(2)             21.90ns     21.90ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(3)                19.44ns     17.32ns
  i64ToStringFollyMeasurePos(3)             17.01ns     17.01ns
  i64ToStringFollyMeasureNeg(3)             21.96ns     21.96ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(4)                20.35ns     18.53ns
  i64ToStringFollyMeasurePos(4)             18.23ns     18.23ns
  i64ToStringFollyMeasureNeg(4)             22.19ns     22.22ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(5)                20.66ns     18.84ns
  i64ToStringFollyMeasurePos(5)             19.47ns     19.54ns
  i64ToStringFollyMeasureNeg(5)             22.66ns     22.68ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(6)                21.89ns     20.19ns
  i64ToStringFollyMeasurePos(6)             20.74ns     20.72ns
  i64ToStringFollyMeasureNeg(6)             23.81ns     23.83ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(7)                23.09ns     21.42ns
  i64ToStringFollyMeasurePos(7)             21.11ns     21.12ns
  i64ToStringFollyMeasureNeg(7)             24.92ns     24.92ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(8)                24.00ns     22.48ns
  i64ToStringFollyMeasurePos(8)             22.18ns     22.18ns
  i64ToStringFollyMeasureNeg(8)             26.20ns     26.20ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(9)                24.31ns     22.94ns
  i64ToStringFollyMeasurePos(9)             23.39ns     23.53ns
  i64ToStringFollyMeasureNeg(9)             26.44ns     26.44ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(10)               25.52ns     24.12ns
  i64ToStringFollyMeasurePos(10)            24.69ns     24.69ns
  i64ToStringFollyMeasureNeg(10)            27.77ns     27.80ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(11)               26.74ns     25.53ns
  i64ToStringFollyMeasurePos(11)            25.07ns     25.07ns
  i64ToStringFollyMeasureNeg(11)            28.87ns     28.87ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(12)               28.25ns     26.59ns
  i64ToStringFollyMeasurePos(12)            26.21ns     26.34ns
  i64ToStringFollyMeasureNeg(12)            30.08ns     30.08ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(13)               29.38ns     26.89ns
  i64ToStringFollyMeasurePos(13)            27.60ns     27.65ns
  i64ToStringFollyMeasureNeg(13)            30.62ns     30.69ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(14)               30.91ns     28.26ns
  i64ToStringFollyMeasurePos(14)            28.58ns     28.57ns
  i64ToStringFollyMeasureNeg(14)            55.27ns     55.27ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(15)               54.71ns     52.06ns
  i64ToStringFollyMeasurePos(15)            52.79ns     52.84ns
  i64ToStringFollyMeasureNeg(15)            57.45ns     57.50ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(16)               57.14ns     54.44ns
  i64ToStringFollyMeasurePos(16)            55.26ns     55.29ns
  i64ToStringFollyMeasureNeg(16)            59.18ns     59.19ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(17)               58.89ns     55.96ns
  i64ToStringFollyMeasurePos(17)            57.10ns     57.14ns
  i64ToStringFollyMeasureNeg(17)            60.89ns     60.88ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(18)               60.33ns     57.69ns
  i64ToStringFollyMeasurePos(18)            58.59ns     58.63ns
  i64ToStringFollyMeasureNeg(18)            62.46ns     62.50ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(19)               62.22ns     59.45ns
  i64ToStringFollyMeasurePos(19)            60.23ns     60.25ns
  i64ToStringFollyMeasureNeg(19)            64.21ns     64.20ns
  -------------------------------------------------------------
  u64ToStringFollyMeasure(20)               63.79ns     60.91ns
  =============================================================

Reviewed By: yfeldblum

Differential Revision: D3455819

fbshipit-source-id: bc57a0e5bd1d1dca22a37c8b7306e05493e6bd5f