Added str::from_slice_with_null() to cast a &[u8] to &str
Added str::subslice_offset()
Renamed slice_bytes_uniqe to slice_bytes_owned
Add example for str::subslice_offset
auto merge of #5823 : Kimundi/rust/str_from_bytes_slice, r=erickt

Added str::from_bytes_with_null
Added str::subslice_offset
Misc cleanup

Closes #5422