Removed unintentional case of "unused variable" warning.

You could get this if you had: `scan!{"x", _ => ()}`.  The internal
`input` variable would be matched, but never used.  Internally, it's
now `_input`.
Changed some use ASTs to use non-global paths.

As far as I can tell, the *only* thing they do is turn back into invalid
code when re-tokenised.
Added scanner! macro.

Closes #7.
Squashed unused import warnings in generated scan code.
Version 0.0.3.