Fix win32 platform race with filesystem::load, add m/pil

This release adds the Platform Isolation Library, which adds an abstraction layer between code and the underlying platform.

The initial release focuses on the Windows system registry, so it's rather Windows focused but in the future it will include filesystem features and be more platform neutral.

Also included is a fix for a race with m::filesystem::load() on win32 when a file changes size while loading.