Better support for &[u16]

[hsivonen]

Currently in rust_icu, APIs that take read-only UTF-16 arguments take &UChar. This means that if the caller has UTF-16 in a different container, the user needs to do something to materialize a UChar container instead. Yet, the underlying C API might be taking UTF-16 by pointer and length.

On the other hand, when the output of an API is UChar, there doesn't appear to be a ready-made way to read its contents as &[u16].

I suggest 1) making UChar deref into &[u16] and 2) making APIs that take read-only UTF-16 arguments take &[u16] (which UChar can then deref to).