In order to use the IdentityMap in the CompilerDispatcher the following
support is added:
 - Support for deleting entries
 - Support for iterating through the entries.
 - Support for AllocationPolicy to enable non-zone allocation of backing
   stores.
 - Also refactors the code a bit.

BUG=v8:5203

Change-Id: I8b616cba8ae9dc22a7f4d76070fbb318c4edc80d
Reviewed-on: https://chromium-review.googlesource.com/444409Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43362}