This is the first step in optimizing Map and Set iterators. This ports
all the base functionality including

 - Set.prototype.entries
 - Set.prototype.values
 - %SetPrototypeIterator%.next
 - Map.prototype.entries
 - Map.prototype.keys
 - Map.prototype.values
 - %MapPrototypeIterator%.next

to C++ and removes all the dead code and the previous half JavaScript
implementation. The next step is to port core parts to CodeStubAssembler
and finally inline the fast-paths into TurboFan directly. The relevant
design document is at:

  https://docs.google.com/document/d/13z1fvRVpe_oEroplXEEX0a3WK94fhXorHjcOMsDmR-8

Most of this work is very similar to how the Array iterator works and we
mostly follow the same process for the implementation.

R=jgruber@chromium.org

Bug: v8:6571
Change-Id: Ieb253d6705ba4077c697a5ff0cb6f87f9c4056ff
Reviewed-on: https://chromium-review.googlesource.com/561138Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46441}