x/sys/windows: add HeapAlloc and related functions

[ericlagergren]

x/sys/windows has LocalAlloc and VirtualAlloc, but not HeapAlloc. HeapAlloc is generally recommended over using LocalAlloc and should be used when you're allocating memory that's not page sized.

Would necessitate HeapCreate and HeapFree.

[networkimprov]

cc @alexbrainman @zx2c4

@gopherbot add OS-Windows

[zx2c4]

What APIs actually require these? Some APIs expect local-allocated memory or return local-allocated memory. But what about the Heap functions?