Optimize JSON escaping of ASCII strings

Optimize JSON escaping of ASCII strings

Summary:
`escapeString` is very slow even when there's very little to nothing to escape. This diff adds a fast path to copy sequences of bytes that don't need escaping.
It also optimizes appending escape sequences: `string::push_back` is slow because it has to do a capacity check for every character.

Before:
```
  ============================================================================
  folly/test/JsonOtherTest.cpp                    relative  time/iter  iters/s
  ============================================================================
  jsonSerialize                                              818.55ns    1.22M
  jsonSerializeWithNonAsciiEncoding                            1.35us  738.06K
  jsonSerializeWithUtf8Validation                              1.42us  705.60K
  jsonSerializeAsciiWithUtf8Validation                         3.27us  306.06K
  parseSmallStringWithUtf                                      1.91us  522.38K
  parseNormalString                                            1.51us  660.27K
  parseBigString                                             384.44ns    2.60M
  toJson                                                     480.54ns    2.08M
  ============================================================================
```

After:
```
  ============================================================================
  folly/test/JsonOtherTest.cpp                    relative  time/iter  iters/s
  ============================================================================
  jsonSerialize                                              781.69ns    1.28M
  jsonSerializeWithNonAsciiEncoding                          847.68ns    1.18M
  jsonSerializeWithUtf8Validation                            928.68ns    1.08M
  jsonSerializeAsciiWithUtf8Validation                       199.85ns    5.00M
  parseSmallStringWithUtf                                      1.93us  518.39K
  parseNormalString                                            1.45us  689.11K
  parseBigString                                             378.66ns    2.64M
  toJson                                                     446.38ns    2.24M
  ============================================================================
```

All string escaping benchmarks are slightly faster, and ASCII-only with no escapes is 8x faster.

Reviewed By: luciang, evilmucedin, yfeldblum

Differential Revision: D4793233

fbshipit-source-id: c40d07708bd787799c8c00f9f23a417b862ee9ae