Add an arena for allocating strings.

[sesse]

This is the patch we took out during the review of PR #2519, now rebased and re-measured. It's much less memory-bad, but still wins ~12%.

Benchmark 1: ./ninja.old -C ~/chromium/src/out/Default content_shell
  Time (mean ± σ):      4.608 s ±  0.014 s    [User: 3.073 s, System: 1.531 s]
  Range (min … max):    4.590 s …  4.634 s    10 runs
 
Benchmark 2: ./ninja -C ~/chromium/src/out/Default content_shell
  Time (mean ± σ):      4.084 s ±  0.009 s    [User: 2.558 s, System: 1.523 s]
  Range (min … max):    4.068 s …  4.097 s    10 runs
 
Summary
  ./ninja -C ~/chromium/src/out/Default content_shell ran
    1.13 ± 0.00 times faster than ./ninja.old -C ~/chromium/src/out/Default content_shell

What do you think? Should we still pursue this?

[sesse]

I moved so that the EvalStrings used in Rule are now owned by a smaller arena on the BindingEnv instead of being allocated on the heap and leaked.

[elliotgoodrich]

I spent a bit of time removing allocations in trimja and the main changes related to EvalString were to:

1. Change EvalString so that its puts all segments in a single std::string so its full capacity is retained when cleared (trimja/evalstring.h)
2. Structure the ManifestParser to reuse the same EvalString throughout parsing (m_storage)

Then the only copies you need to make of EvalString is in Rule::AddBinding, which would be faster as the copy constructor of trimja's EvalString only needs 1 allocation and copy of contiguous data. Otherwise, during parsing the capacity is reused in the single EvalString and eventually we'll stop any allocations.

Maybe changing the entire ManifestParser is too much work, but it looks possible to add an EvalString member variable and reuse this?

[elliotgoodrich]

I've submitted 1. here #2696

The second part would need a ReusableStack<EvalString> that doesn't actually destruct elements when clearing and will reuse them later on. This will replace the vector<EvalString> inside ManifestParser to remove the remaining allocations in this area.