* update support for uts39 from unicode 15
* follow uts39's recco that it's not necessary to require
idents to be single-script (they call out proglang idents,
reference the new uts55-5). We use a heuristic derived from
the concept of identifier chunks from uts55-5, to allow
idents like foo_bar_baz where each chunk around the _ can be
single-or-highly-restrictive
* provide directional confusability detection, by reversing
spans of direction-changed chars in idents for bidi_skeleton,
see issue #12929
* nfc by default
* suggest nfkc on unexpected tokens when nfkc would have worked
* support additional normalizations (just micro/mu for now)
* parser, formatter tests
* tweak Macro.inner_classify(atom) not to rely on not_nfc causing error