allow approximating unions of numeric types

allow approximating unions of numeric types

 * Binding *APPROXIMATE-NUMERIC-UNIONS* does that. It must be bound
   only by callers of TYPE-UNION that know what they want -- in general

     (OR (INTEGER 1 2) (INTEGER 3 4)) => (INTEGER 1 4)

   is wrong, as (NOT (INTEGER 1 4)) doesn't include 3. But in special cases
   like deriving the return type of a function it can be done.

 * Rename MAKE-CANONICAL-UNION-TYPE MAKE-DERIVED-UNION-TYPE, and bind *A-N-U*
   there if we start accumulating an overly large union of numeric types.
   Definition of "overly large" can be adjusted via
   *DERIVED-NUMERIC-UNION-COMPLEXITY-LIMIT*.

 * Fixes lp#309448 and the recent compiler performance regression due
   to new CONCATENATE deftransform as reported on sbcl-devel.