1.0.12.13: sequence optimizations: SUBSEQ, part 3

1.0.12.13: sequence optimizations: SUBSEQ, part 3

* Split the optimized data-vector accessor fetching logic into a
  global macro, so that sequence functions can fetch the appropriate
  setter/getter just once, instead of doing the dispatch per access.

* Use this to optimize VECTOR-SUBSEQ*.