UTF16String API updates

[stevengj]

In preparation for JuliaLang/julia#7016 or similar changes, you should replace UTF16String(data) by utf16(data) and length(s.data) by sizeof(s) >> 1.

[PallHaraldsson]

I see "length" "-1" in the code, is this maybe already fixed (in a different way)? Was this forgotten or want to use size? For performance reasons? Length would have to scan the whole string (in case of non-BMP).

[stevengj]

Yes, looked like @nolta fixed it in 0cc11e2 without cross-referencing this issue.

The only downside to using length(s.data)-1 is that it depends on the internal representation of UTF16String, so if the NUL-termination is ever removed this will have to change again.