Get rid of tuple_field_by_part_multikey

Always require multikey_idx to be passed to tuple_field_by_part.
If the key definition isn't multikey, pass -1. Rationale: having
separate functions for multikey and unikey indexes doesn't have
any performance benefits (as all those functions are inline), but
makes the code inconsistent with other functions (e.g. tuple_compare)
which don't have a separate multikey variant. After all, passing -1
when the key definition is known to be unikey doesn't blow the code.
While we are at it, let's also add a few assertions ensuring that
the key definition isn't multikey to functions that don't support
multikey yet.