1.0.17.31: more constant cleverness

 * Make MAYBE-EMIT-MAKE-LOAD-FORM can dump _all_ references to
   non-trivial named constants using the name (well, not FP constants
   for SBCL itself.)

   This means that after (DEFCONSTANT +FOO+ "FOO") all references to
   +FOO+ are EQ, even in different files.

   ...some people are going to use this as an unportable performance
   hack, and their code will break horribly sooner or later, but more
   importantly we need to grovel less things, and more sharing means
   less memory use and better cache behaviour.

 * Tests.