Fix `showPattern` to round-trip

[andreasabel]

Better docs for Pattern module (fixed in 1.3.2), fix showPattern to round-trip.

It is claimed that showPattern round-trips, but does not:

>>> getAllTextMatches ("a \" \\" =~ "[a]") :: [String]
["a"]

>>> let parse = either (error . show) fst . parseRegex
>>> let pp' = showPattern . parse
>>> getAllTextMatches ("a \" \\" =~ pp' "[a]") :: [String]
["a","\""]

It uses Show PatternSet which adds " coming from empty components of PatternSet:

regex-tdfa/lib/Text/Regex/TDFA/Pattern.hs

Lines 102 to 109 in c5b8f54

	showsPrec i (PatternSet s scc sce sec) =
	let (special,normal) = maybe ("","") ((partition (`elem` "]-")) . Set.toAscList) s
	charSpec = (if ']' `elem` special then (']':) else id) (byRange normal)
	scc' = maybe "" ((concatMap show) . Set.toList) scc
	sce' = maybe "" ((concatMap show) . Set.toList) sce
	sec' = maybe "" ((concatMap show) . Set.toList) sec
	in shows charSpec
	. showsPrec i scc' . showsPrec i sce' . showsPrec i sec'