This replace HeapType with a dedicated class that implements just what we need for field type tracking. In the next CL, I plan to remove FieldType::Iterator because FieldType can iterate over at most one map.

The ultimate plan is to get rid of templates in types.(h|cc) and remove type-inl.h.

TBR=rossberg@chromium.org

Review URL: https://codereview.chromium.org/1636013002

Cr-Commit-Position: refs/heads/master@{#33521}