1.0.2.1: DATA-VECTOR-{REF,SET}-WITH-OFFSET for the x86

1.0.2.1: DATA-VECTOR-{REF,SET}-WITH-OFFSET for the x86

	Compile calls of (AREF FOO (+ INDEX <constant>) more efficiently:
	... turn DATA-VECTOR-{REF,SET} into
	  DATA-VECTOR-{REF,SET}-WITH-OFFSET when the element type of FOO
	  is at least 8 bits wide;
	... introduce general mechanism for optimization of such calls;
	... redo the x86 DATA-VECTOR-FOO VOPs, reducing the number of such
	  VOPs in the process;
	... do the same for BIGNUM-REF and SAP-REF-FOO.

	Upshot: 5-10% increase in performance on array-heavy code such
	  as Ironclad; a 20% increase in performance has been observed
	  on cellular automata codes.  Some restrictions apply; see the
	  KLUDGE in src/compiler/generic/vm-tran for an example.