Many, many tweaks to the lexer

This is the closest comparison between rustc and the model, I think.
Merge pull request #5 from cmr/master

Lexer changes
Merge pull request #6 from ptgreen/master

Enhancements to LALR parser and rparse, applied after cmr's not-yet-landed work.
Re-remove #include "tokens.h" to permit reusing lexer between parsers.

Note: the proper token definition is fed in by the make rules
explicitly using "clang -include".
Update divergent token sets between tokens.h and parser grammars.
Fix printf warning.
Fix LDFLAGS so lexer links (needs -lm for pow()).
De-duplicate and correct view-item / use rules.
Re-factor struct_expr.
Enable all-errors warning mode on bison.
Fix useless precedences and fix the various path forms (documenting context).
Fix empty rules and remove useless rules. Builds with error=all now.
Add item_type.
Add item_trait.
Add item_impl.
Flesh out pat, add paths to exprs.
Flesh out ty, add path_expr permitting leading MOD_SEP.
Merge pull request #7 from ptgreen/build-fixes

Build fixes
Merge pull request #8 from ptgreen/misc-lalr-enhancements

Misc LALR enhancements
Expand action rules in LALR parser.