Another go-round with resolution of ambiguous functions and operators.

In function parsing, try for an actual function of the given name and
input types before trying to interpret the function call as a type
coercion request, rather than after.  Before, a function that had the
same name as a type and operated on a binary-compatible type wouldn't
get invoked.  Also, cross-pollinate between func_select_candidates and
oper_select_candidates to ensure that they use as nearly the same
resolution rules as possible.  A few other minor code cleanups too.