Add lint for type-checking transmutes

@Ms2ger wanted this. It's set to `Allow` so that we don't get inundiated with warnings.

Note that Cargo hides warnings when compiling dependencies. You can fix this by using `Deny` instead, and `#[allow(transmute_type_lint)]` on audited transmutes.

I'll work on a fix for Cargo to not capture warning output when I get time.