Made a fexl executable.

Added parsing of symbolic forms enclosed in braces.

Simplified the show routine and how it can be extended.

Added many extensive tests, including syntax errors, undefined symbols, and
pathological run-time behavior such as exceeding limits on time, memory, and
stack.

Renamed "atom_" routines to "get_".

Include directives now alphabetized, except where something needs to be pulled
ahead of the others.

Eliminated separate "atom" routine.  It's just clearer to use the V-form
directly.

Added type_long.