const `ByteSubstring::new`

[jonathanstrong]

hey - looks like this was at least planned at some point, since /* const */ keyword was commented out in a few places.

are you interested in merging something like this?

seems like the const version of the simd code is a bit uglier, but equivalent. if there are concerns about the const version of the code in new not being as fast, perhaps there could be a impl<'static> ByteSubstring<'static> block with a new_static method or something.