Provide original encoding for string constants

[kennykerr]

Now that the win32 metadata captures original string encoding (microsoft/win32metadata#1008), we can use the s! and w! macros to provide those string constants in their original encoding. This makes many string constants directly and efficiently usable as constants to pass to various Windows APIs that expect either Ansi or Wide null-terminated string literals. For example, you can now call the BCrypt APIs with the various string constants directly:

let mut rng = Default::default();
BCryptOpenAlgorithmProvider(&mut rng, BCRYPT_RNG_ALGORITHM, None, Default::default())?;

I have also taken the liberty to separate the w! macro for wide string literals from WinRT support and now provide a dedicated h! macro for compile-time HSTRING support. This simplifies the vast majority of wide string literals that don't need WinRT support.