`as_bytes` for CString16/CStr16

[RaitoBezarius]

Hi there, it seems like there's a small API impedence mismatch with set_variable which takes bytes and CStr16/CString16 which can only converts back to &[u16], it would be interesting to plug it "immediately" as bytes: &[u8].

[phip1611]

Good catch! Thanks. I've set up #788 for that.