Some minor improvements.

- Add a new ASSERT-NIL assertion form.  This is, of course, equivalent to ASSERT-FALSE.  But names matter; without knowing your API in full detail I found myself wanting to write ASSERT-NIL to check that a list of results is NIL, but I had to use ASSERT-FALSE, which is (psychologically) not the same thing (even though for the purpose of evaluating values logically falsity = NIL).
- Clean up the various specializations on the RECORD-FAILURE generic function.  On CCL 1.9, this particular practice of using CALL-NEXT-METHOD was leading to errors (specifically, the list of applicable methods was changing because the first argument was going from a known symbol [via the EQL specializer] to an unknown symbol, i.e., the list of applicable methods was changing from a list of length 2 to a list of length 1.
- In .gitignore, ignore fasls generated by a (64-bit) CCL.
- Use HASH-TABLE-P to do some sanity checks on the tags and test tables.
- Trim some unnecessary end-of-line whitespace.