Fix argument to SAY.

Whitespace.
Various breaking changes

Many optional arguments are no longer optional.
It's less convenient, but more predictable.

The excess responsibilities of CHALLENGE are split
into PROCESS-ANSWER.

PRACTICE is replaced with the simpler and more apt GRIND.

Practice list construction is refactored.
MAKE-PRACTICE-TABLE in particular now serves as an unambiguous
top-level function.