match: add exact flag to match() to unify all match forms

match: redefine always and never in terms of match and exact

match: fold _globprefix into _roots

match: optimize escaping in _globre - localize re.escape - fastpath escaping of non-special characters

match: remove head and tail args from _globre

match: fold _matcher into match.__init__

match: rename _matchfn to _buildmatch

match: optimize _patsplit

match: tweak some names

match: simplify _matcher - get rid of special case - simplify anypats logic - fold inckinds and exckinds

match: split up _normalizepats

match: optimize _globprefix

match: unnest functions in _matcher

match: kill unused defaults on _globre

match: kill test in matchfn

match: refactor matchfn generation