Test #7484 in th/T7484
Fix #7484, checking for good binder names in Convert.

This commit also refactors a bunch of lexeme-oriented code into
a new module Lexeme, and includes a submodule update for haddock.
Test #1476 in th/T1476
Test that nested pattern splices don't scope (#1476).

Test case: th/T1476b.
Fix #1476 by making splice patterns work.

Unfortunately, splice patterns in brackets still do not work
because we don't run splices in brackets. Without running a pattern
splice, we can't know what variables it binds, so we're stuck.

This is still a substantial improvement, and it may be the best
we can do. Still must document new behavior.
Release notes for #1476, #7484.
Test #9824 in th/T9824
Fix #9824 by not warning about unused matches in pattern quotes.
Update manual for pattern splices (#1476)