Beginnings of trying to upstream Windows changes

I've deleted a couple kloc from my fork that's going to be too complicated to try to upstream [1]. The remaining diff [2] is somewhat more tractable, hopefully.

This pull is an attempt at picking some arbitrary subset that's somewhat separable to try to make some progress.

The majority of the full diff is to store a mmap'd file that's a map from filename->deplist. So, this is most of the deplist.

I know it's still kind of huge. If you see something else that looks like a better place to try to start, I'm game for trying something else too.

1. https://github.com/sgraham/ninja/compare/with-stat-daemon
2. https://github.com/sgraham/ninja/compare/master