UTF-16 string types

This crate provides two string types to work with UTF-16 encoded bytes, they are directly analogous to how String and &str work with UTF-8 encoded bytes.

UTF-16 can be encoded in little- and big-endian byte order, this crate identifies which encoding the types contain to using a generic byteorder type, thus the main types exposed are:

• &WStr<ByteOrder>
• WString<ByteOrder>

These types aim to behave very similar to the standard libarary &str and String types. While many APIs are already covered, feel free to contribute more methods.