tiny fbstring push_back(Char) optimization

tiny fbstring push_back(Char) optimization

Summary:
I noticed that push_back(Char) was slower for fbstring than
for std::string for long strings and found that I could make
it faster by inlining the call to mutable_data() and
exploit that it's always non-small and non-shared.

Benchmarks before:

./folly/test/FBStringTestBenchmarks.cpp.h       relative  time/iter  iters/s
BM_push_back_string(1)                                      69.42ns   14.41M
BM_push_back_string(23)                                    582.31ns    1.72M
BM_push_back_string(127)                                     1.47us  682.12K
BM_push_back_string(1024)                                    5.52us  181.07K
BM_push_back_fbstring(1)                                     9.55ns  104.74M
BM_push_back_fbstring(23)                                  212.45ns    4.71M
BM_push_back_fbstring(127)                                 864.00ns    1.16M
BM_push_back_fbstring(1024)                                  6.73us  148.52K

and after:

BM_push_back_fbstring(1)                                     9.55ns  104.74M
BM_push_back_fbstring(23)                                  212.45ns    4.71M
BM_push_back_fbstring(127)                                 783.08ns    1.28M
BM_push_back_fbstring(1024)                                  4.03us  248.05K

Test Plan: fbconfig folly/test && fbmake runtests

Reviewed By: tudorb@fb.com

FB internal diff: D646081