Improve `str` prefix/suffix comparison
Improve `str` prefix/suffix comparison

The comparison can be performed on the raw bytes, as the chars can
only match if their UTF8 encoding matches.

This avoids the `is_char_boundary` checks and translates to a straight
`u8` slice comparison which is optimized to a memcmp or inline
comparison where appropriate.

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust