Fixed a couple of lexer/parser errors in flex mode (-F option).

Fixed a couple of lexer/parser errors in flex mode (-F option).

This fixes bug #229: re2c option -F (flex syntax) broken,
reported by Robert van Engelen.

A well-formed example that caused syntax error (flex-style raw literal
followed by one or more spaces and a curly brace):

/*!re2c
    a {}
*/

The faulty behaviour goes back as far as re2c-0.13.6 (and supposedly
before that): in flex mode, raw literal may occur in various contexts
both as a regexp (string literal) and an identifier (named definition,
condiiton name). RE2C uses lookahead to infer the context and determine
the appropriate type of lexer token, but it missed some cases.

The fix has two sides. First, if reduces the number of contexts where
the general lexer may encounter raw literal (by using specialized lexers
for condition lists <x,y,...,z> and condition goto => and :=>). Second,
it fixes the lookahead regexps used for context inference.

Also added a bunch of tests (generated by a script).