Add flexible registry type and byte query support

[kennykerr]

The windows-registry crate is currently pretty strict when it comes to mapping registry values and types. You cannot for example read the bytes from a registry value unless the type of that value is REG_BINARY. This makes it a little difficult to gracefully deal with invalid registry values.

This update adds the get_type methods and the resulting Type enum so that the actual type of a value can be queried directly. It also updates the get_bytes method to support returning the raw byte value even if the type isn't known to be bytes.

Related: #3119

[ChrisDenton]

Seems good to me. Arguably we shouldn't allow setting non-unicode registry values, even if we can read them, but it is convenient to be able to use OsStr and I don't feel there's a strong reason to implement checks.

[kennykerr]

Thanks, I'll work on OsStr support separately.