make_ascii_lower/uppercase for Windows OsString?

[TyPR124]

The standard library seems to assume calling make_ascii_uppercase() on the underlying [u8] is safe on Windows, seen here:

rust/src/libstd/sys/windows/process.rs

Lines 35 to 41 in 0eb878d

	impl From<OsString> for EnvKey {
	fn from(k: OsString) -> Self {
	let mut buf = k.into_inner().into_inner();
	buf.make_ascii_uppercase();
	EnvKey(FromInner::from_inner(FromInner::from_inner(buf)))
	}
	}

Would it be reasonable to add a make_ascii_uppercase() and/or make_ascii_lowercase() to Windows OsStrExt and/or OsStringExt to make this available to users of std?

Even if the internal representation of OsString changes down the road, I would assume the code above would still need to be replicated with the new representation, so it seems like it should be okay to have make_ascii_uppercase a stable/public API.

If this is a reasonable request, I am willing to make a PR with the appropriate methods implemented in the appropriate places.

[CryZe]

Considering OsString is a superset of UTF-8, make_ascii_* should work across all platforms.