Arity check types and functions (->)

[janmasrovira]

This pr fixes the following:

This example causes the compiler to crash with "implicitness mismatch".

f : id Bool -> Bool;
f _ := false;

The reason is that id expects an implicit argument but finds Bool, which is explicit. The arity checker was not inserting any hole because it was ignoring the whole type. Moreover the aritychecker was never checking -> types as we never expected to
have to insert holes in -> types (since the only fragment of defined functions that we accept in types are those which do not have implicit arguments).

We now properly arity check all types and process the function type -> correctly.