Support exposing &[u16] as std::u16string_view and u16 as char16_t

[hsivonen]

In text-oriented Rust code like ICU4X, &[u16] is used for UTF-16 and, therefore, has std::u16string_view semantics, and u16 is used for UTF-16 code unit and, therefore, has char16_t semantics. Add configuration options to have these types generated for C++.

[Manishearth]

We definitely use u16 for non char16_t reasons in ICU4X, fwiw

Slices are fine though

[robertbastian]

#367 #368