Several optimizations for R6RS fixnum arithmetic

Several optimizations for R6RS fixnum arithmetic

* module/rnrs/arithmetic/fixnums.scm (assert-fixnum): Is now a
  macro.
  (assert-fixnums): New procedure checking a the elements of a list
  for fixnum-ness.  All callers applying `assert-fixnum' to a list
  now changed to use this procedure.

* module/rnrs/arithmetic/fixnums.scm (define-fxop*): New for defining
  n-ary inlinable special-casing the binary case using `case-lambda'.
  All applicable procedures redefined using this macro.

* module/rnrs/arithmetic/fixnums.scm: Alias all predicates to
  their non-fixnum counterparts.