Match var hygiene, etc

This PR includes two big things and a bunch of little ones.

1) It enables hygiene for variables bound by 'match' expressions.
2) It fixes a bug discovered indirectly (#15221), wherein fold traversal failed to visit nonterminal nodes.
3) It fixes a small bug in the macro tutorial.

It also adds tests for the first two, and makes a bunch of small comment improvements and cleanup.