Type resolution for operators

I've committed changes to parse_oper.c to enable fallback to string type
when argument(s) are of UNKNOWN type. This is the same code (verbatim)
as I recently added for function resolution.

An obvious example is for

select '1' = '01';

which used to throw an error and which now resolves to two text strings
(and returns 'false'). To force these to be handled as integers, prefix
one with the "int" type specifier:

select int '1' = '01';

which, btw, returns 'true'.

Regression tests pass without change (which I guess means that we don't
have good coverage there, either :/

- Thomas