This commit improves the diagnostics for vector (elementwise) builtins
in a couple of ways.
It primarily provides more precise type-checking diagnostics for
builtins with specific type requirements. Previously many builtins were
receiving a catch-all diagnostic suggesting types which aren't valid.
It also makes consistent the type-checking behaviour between various
binary and ternary builtins. The binary builtins would check for
mismatched argument types before specific type requirements, whereas
ternary builtins would perform the checks in the reverse order. The
binary builtins now behave as the ternary ones do.
