Clarify language spec regarding overloads

[JimLarson]

Overloads must be disjoint when computed on runtime types. They can have finer-grained types at check time, but must not overlap when types coarsen to the runtime times. Can have type variables at either runtime or check time, but cannot violate the previous two rules.

For instance, consider a function same_value(map<A, B>, A, A) --> bool which returns whether two map keys have the same value. The runtime type pattern for this must be (map, A, B) --> bool with independent type variables A and B, because the check-time notion of "same type" could include dyn, and each instance of dyn should coarsen to a fresh type variable.