folly::toJson() performance

[miloyip]

While working on nativejson-benchmark, I found that the runtime performance of folly::toJson() is extremely low for big JSONs (OS X 10.11.6, clang7.0, 6a6ac91):

After profiling, it is found that the time killer is in:

// json.cpp
void escapeString(
    StringPiece input,
    std::string& out,
    const serialization_opts& opts) {
  auto hexDigit = [] (int c) -> char {
    return c < 10 ? c + '0' : c - 10 + 'a';
  };

  out.reserve(out.size() + input.size() + 2); // <-- this line
  out.push_back('\"');
  // ...

This line causes O(n^2) time complexity, because every time it expands just fit for the string to be appended. This may be only the behavior of libc++. And C++ says:

If new_cap is greater than the current capacity(), new storage is allocated, and capacity() is made equal or greater than new_cap.

The std::string::push_back() and std::string::append() expand double (or other multiples) of the current capacity, which makes amortized O(n).

After removing that reserve() call, the result looks much more normal:

I will prepare a pull request on this.