Moves the hashmap's allocator from being a parameter in the various
hashmap functions, to being a field in the hashmap itself. This

1. Protects against incorrectly passed allocators, and
2. Cleans up the API so that e.g. callers don't have to store their
   allocator

This is part of a wider set of changes discussed in:
https://groups.google.com/forum/#!topic/v8-dev/QLsC0XPYLeM

Review-Url: https://codereview.chromium.org/2345233003
Cr-Commit-Position: refs/heads/master@{#39538}