- lexer.rl: correctly report the %r%% tREGEXP_BEG value as %r%.
- lexer.rl: correctly disambiguate ident!= as tIDENTIFIER, tNEQ.
- lexer.rl: correctly disambiguate "x ::Foo" as tIDENT, tCOLON3, ...
- lexer.rl: emit tCONSTANT/tIDENTIFIER/tFID in expr_dot.