1.0.18.1: correct handling of SATISFIES types in the compiler

1.0.18.1: correct handling of SATISFIES types in the compiler

 * CTYPEP used to retur a secondary value of true in cases where the
   function in question was not foldable.

 * Slightly sleazily extent SB-C::CONSTANT-FUNCTION-CALL-P (part of
   CONSTANTP) to return the primary result of the call as the
   secondary value, so CTYPEP can use it.

 * Test-case.