Highlight strings more properly in Rust.
Highlight strings more properly in Rust.

- Raw strings are now ``String`` rather than the unofficial ``String.Raw``
  which isn?t styled by default;

- Byte strings weren?t being handled at all;

- ``\x80`` and above aren?t valid in non-byte strings;

- ``\0`` is a thing, ``\777`` for octal 7 is *not* a thing, nor has it ever
  been, to the best of my knowledge. Not sure why that one was in there at all.

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