Change the documentation.

code that uses them is correctly indented by editors.

with ~@<newline> format directives.

test/unification-tests.lisp: new test to verify the change

Make (asdf:test-system 'cl-unification) work.

before the forms are read. This fixes compilation of
file unification-tests.lisp.

I'd like to tell you this is part of a plan to simplify %match-expander, but in
all honesty, the only reason I'm doing it is because I found myself thinking
with-unification-variables would be a handy macro while working on a private
extension to cl-unification, and I didn't want to duplicate the code.

Because (in-readtable ...) is a beautiful thing.

Useful when there are conflicts on #T, such as with closure-html.

And, because it's now trivial, add MATCH-COND, MATCHF-COND, MATCH-ECASE, and
MATCHF-ECASE.

Still to do: Merge documentation of functions so there's less copy-pasta in the
documentation strings.

...why is this not default behavior?

Let errors which are not UNIFICATION-FAILUREs find their way out of UNIFY*.

If the first clause doesn't match due to mismatched case, it would
roll down to the second clause, causing case-differing strings to
erroneously match. By checking for case again, we can avoid this.

Note, however, that this does not fix the similar but unrelated
inconsistent usage of *unify-string-case-sensitive-p* and
*unify-string-case-insensitive-p* (the former being used in the code
and the latter appearing in documentation). That bug will remain
until the canonical form is decided upon.

This both greatly simplifies the MATCH-CASE macro as well as its expansion.

HOWEVER, this version is *NOT* 100% compatible with the previous version.
Specifically, UNIFICATION-FAILUREs signalled from within clause-forms will /not/
cause the next unification clause to be attempted, but will instead propogate
outward as the -case name suggests they should.

That is,
(ignore-errors
(match-case ("foo")
("foo" (error 'unification-failure ...))
(t :default)))
=> :default ;; before patch
=> nil, #<unification-failure> ;; after patch

Because of the way MATCHING expands, and what UNIFY* returns, each
(setf #:env (unify* ...))
call will do one of two things: it will set #:env to NIL or it will set #:env to
an ENVIRONMENT structure.

If #:env is set to NIL--the same value it entered the (setf) with!--the COND
will continue on to the next clause.

If #:env is set to an ENVIRONMENT structure, none of the remaining (setf)
clauses will be evaluated.

Thus, because the variable will only ever be set to a non-nil value once, this
should be perfectly safe.

Rather than skipping to the next clause on any error, UNIFICATION-FAILUREs--and
/only/ UNIFICATION-FAILUREs--skip to the next clause.