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.

src/test/regress/expected/strings.out

@@ -17,7 +17,7 @@ SELECT 'first line'
 ' - next line' /* this comment is not allowed here */
 ' - third line'
 	AS "Illegal comment within continuation";
-ERROR:  parser: parse error at or near "'"
+ERROR:  parser: parse error at or near "' - third line'"
 --
 -- test conversions between various string types
 --