vlist-cons micro-optimizations

vlist-cons micro-optimizations

* module/ice-9/vlist.scm (set-block-next-free!): Define this instead of
  increment-block-next-free!.
  (block-append!): Refactor to take an offset, and only append if the
  offset is the next free value, and there is space in the block.
  (block-cons): Refactor to not be a loop.  The partial evaluator would
  have to understand effects analysis in order to be able to unroll it,
  and there's at most one recursion.

  Recovers the performance loss resulting from the previous commit.