Spelling fix and improve comments
Avoid storing explicit NIL as docstring. Use CLEAR-INFO instead.
OUTPUT-FDEFN should be intelligible despite print-level/length cutoffs.
Simplify unquoting COMMA make-load-form in target image.
Use WITH-TEST throughout tests/walk.impure.lisp

Also, don't manually print offending values in STRING=-MODULO-TABSPACE
since ASSERT does that automatically nowadays.