Add a stat cache. Demo-quality, and disabled atm.
Turn on stat cache. Empty builds of chrome on my laptop 4s -> 1.3s (!)
minor cleanups
more minor cleanups
more minor cleanups
simplify more, move behind flag
make win-only
comment
on by default
error checking
Free cache memory once it's no longer used.

Doesn't slow down empty build times measurably, and
saves some memory on non-empty builds.
add some statcache tests
make bool exist everywhere, for simpler calling code
add missing _WIN32 checks
s/hFind/find_handle/
Merge pull request #779 from nico/statcache

Use FindFirstFileEx() for stat()ing files on Windows.