a) The current size of the backing store for the array under splice
    wasn't considered. Additionally, allocate the array with the
    normal growing strategy.

b) Use primitives memcpy and memmove when
   appropriate. These calls are wrapped in new CSA functions
   MoveElements and CopyElements, which use the C functions when a
   write barrier isn't needed (otherwise they just copy elements
   in a loop).

Bug: chromium:880780
Change-Id: I39a917c71036f52250c68f2cced77a1c24f97b67
Reviewed-on: https://chromium-review.googlesource.com/c/1243104
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56534}