Add header pruning

[Jake-Shadle]

The Windows SDK has ~315MiB of headers (and other junk) under include, and similarly to #84, only a small fraction will ever be used by a project. Header pruning will probably be a little more involved if one wants to remove headers transitively, but with a little effort one should be able to remove massive swathes of includes that any single project never uses, potentially reducing disk (container) usage quite a lot.

Also, the winrt directory is 121MiB alone, and most C/C++ code that is using Windows is ancient and thus doesn't know nor care about WinRT, so that should at least be an easy win for most projects.