Adds template parameters for the TemplateHashMapImpl for the key and
value type, to allow them to be something other than pointers. To keep
the impact of this patch low, uses of TemplateHashMapImpl set these
types to void* to emulate the previous behaviour.

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/2343123002
Cr-Commit-Position: refs/heads/master@{#39530}