check types in clause heads

[mndrix]

Imagine a predicate defined as follows:

%% something(X:integer)
something(foo(_)).

There's an obvious disagreement between the documented types (integer) and the clause head (foo(_)). Here is a process for discovering these kinds of problems:

• call is_of_type(integer, foo(_))
  • if success, the clause head is OK
• generate random values of integer (using quickcheck:arbitrary/2) and try unifying them with foo(_)
  • if we find a single value that unifies, the clause head is OK
• clause head is almost certainly wrong; show warning

We should export a predicate which answers the question "is this value possibly compatible with this type?" That step in the process seems like it might be useful elsewhere.