In the process, re-factor the implementation of Array.prototype.forEach so that
the bulk of the implementation can be re-used, since much of the spec is
identical. The refactor should also make it more straight-forward to implement
map and filter. The re-factored version only have a single slow path for processing
elements which is used for both the overall slow path and for the bailout from the
FAST_ELEMENTS case.

Review-Url: https://codereview.chromium.org/2709773002
Cr-Commit-Position: refs/heads/master@{#43745}