Improve lexer's error reporting. You get the whole token mentioned now

Improve lexer's error reporting.  You get the whole token mentioned now
in parse error messages, not just the part scanned by the last flex rule.
For example,
	select "foo" "bar";
used to draw
	ERROR:  parser: parse error at or near """
which was rather unhelpful.  Now it gives
	ERROR:  parser: parse error at or near ""bar""
Also, error messages concerning bitstring literals and suchlike will
quote the source text at you, not the processed internal form of the literal.