Optimize CHAR-EQUAL on constant and base-char args.

The open-code transform for base-char arguments was never invoked, it
should have been defined on TWO-ARG-CHAR-EQUAL, not CHAR-EQUAL. And
enable it only for (> speed space).

Add a check for base-char into the TWO-ARG-CHAR-EQUAL function, and
invoke the optimized code, the same the transform uses.

Optimize (char-equal #\c x) by transforming it into a call to
(char-equal-constant x #\c #\C), which does
(or (char= #\c char) (char= #\C char)), or directly to that expression
with (> speed space).

(char-equal #\- x) is transformed to (char= #\- x).