Provide a namespace.c function for lookup of an operator with exact

Provide a namespace.c function for lookup of an operator with exact
input datatypes given, and use this before trying OpernameGetCandidates.
This is faster than the old method when there's an exact match, and it
does not seem materially slower when there's not.  And it definitely
makes some of the callers cleaner, because they didn't really want to
know about a list of candidates anyway.  Per discussion with Atsushi Ogawa.