Refactor: simplify lexing of the dot

Before this patch, the lexer did a truly roundabout thing with the dot:

1. look up the varsym in reservedSymsFM and turn it into ITdot
2. under OverloadedRecordDot, turn it into ITvarsym
3. in varsym_(prefix|suffix|...) turn it into ITvarsym, ITdot, or
   ITproj, depending on extensions and whitespace

Turns out, the last step is sufficient to handle the dot correctly.
This patch removes the first two steps.