documented :stxclass patterns
  closes PR 13565
change syntax-parse clause parsing code
add #:commit, fix other typos
basic pattern factoring for syntax-parse
do factoring for stxclasses (non-splicing only) too
unfold pat:and to uncover more factoring opportunities

Side clauses introduce pat:and, very common in stxclasses.
improve pattern factoring, misc cleanup
split out pattern optimization code
fix "one term at a time" mode for lifts
  closes PR 13597
reorganize code, expose stxclass rhs->parser helper