0.9.10.17: fix bug #400, aka more correct CONSTANTP

0.9.10.17: fix bug #400, aka more correct CONSTANTP
 * To know that a function call is a constant we need to check that
    it doesn't signal an error in addition to knowing that the function
    is foldable.
 * To know that a THE form is constant we need to check that the
    value is of the right type and be prepared to deal with malformed
    type specifiers.
 * Tests, tests, tests.

 ...all in all, I'm starting to miss the evaluator branch.