1.0.31.11: better handling of vector types in LOOP

1.0.31.11: better handling of vector types in LOOP

 This is really 1.0.26.12 in a fixed form.

 * LOOP-DECLARE-VAR calls LOOP-TYPED-INIT iff there is no explicit
   initialization form, and when it does call it, the type is
   constructed from the result using TYPE-OF.

 * LOOP-TYPED-INIT knows how to construct zero-length vectors for all
   reasonable vector types (ones expressible with an ARRAY-TYPE.)

 * LOOP-MAKE-VAR informs LOOP-DECLARE-VAR about user-provided
   initialization.