More Win32 metadata now employs a const attribute to indicate const parameters and return types, so we can now use that to apply const more uniformly. Since Rust distinguishes between const and mut pointers, I added distinct to_const_type and to_const_ptr type transformations to more clearly distinguish between what is actually modified in different instances.

With this update, the fix for microsoft/win32metadata#1454 should now flow automatically into windows-rs.