[heap] Move IdentityMap data structure out of heap.

[heap] Move IdentityMap data structure out of heap.

This data structure uses the public heap API only and is not specific
to any heap internals. It should be usable throughout V8 and inclusion
of the header file should not be restricted.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/1320503004

Cr-Commit-Position: refs/heads/master@{#30443}