Add helper for strings

[piegamesde]

Usually, one does not want to check individual characters, but strings as a whole. Since this results in repeating the same boilerplate over and over, how about providing a direct mechanism on &str for this purpose? I'm thinking of three methods: starts_with_xid_start, is_xid_continue, is_xid_ident. I'm fine with them being either standalone methods or trait extensions to str.

[lunabunn]

What benefit does s.is_xid_continue() provide over the only slightly longer s.chars().all(UnicodeXID::is_xid_continue)?

[piegamesde]

s.chars().all(UnicodeXID::is_xid_continue) is not only twice as long, which I would not call insignificant, it is also less concise to read (one mental operation vs three).