A from_str(_radix) analogue

Working with mostly-UTF-8 byte strings, the two biggest things my current project is missing are substring matching (which this crate provides) and integer parsing (which std provides only for str).

This is easily worked around by calling str::from_utf8 first, because if the input is not valid UTF-8 then it would never make it through a from_str/parse or from_str_radix anyway.

But it would be convenient to have these functions in bstr as well, to avoid the overhead from that redundant from_utf8 call.