Fewer strEQ calls in toke.c:S_new_constant

Fewer strEQ calls in toke.c:S_new_constant

There is a small fixed number of keys that can be passed to this
static function:

charnames
binary
float
integer
q
qr

In a few places, we check whether the key is "charnames".  It would
be quicker just to check the first character, since, if it is 'c', the
key must be "charnames".  (Under debugging builds, assert that that
assumption is true.)