Enforce types returned from CLOS stream-read-char (etc) methods.

As per discussion on sbcl-devel, these are necessary as a consequence
of change 7cdaadca removing checks from callers of READ-CHAR et. al.
Revert "Derive that (INTERN x "KEYWORD") produces a KEYWORD"

This reverts commit 1d796c22c86e8be5e49857edb3637cc9b70d0d94.