1.0.14.28: small FGEN improvements

1.0.14.28: small FGEN improvements

 * Use a DEFSTRUCT instead of a vector for clarity. Also eliminate the
   unused mystery slot.

 * Inline code of GET-NEW-FUN-GENERATOR-INTERNAL in
   GET-NEW-FUN-GENERATOR.

 * Store in a hash-table for O(1) goodness instead of a list.

 * ENSURE-FGEN instead of MAKE-FGEN + STORE-FGEN.

 * When lifting constants out of code in FGEN construction, leave
   constant lists in place: several of the functions we generate FGENs
   for have ETYPECASE forms, which will otherwise cause us to close
   over the :EXPECTED-TYPE and :POSSIBILITIES arguments to ERROR for
   no good reason.