The big change here is to split out each key type into its own
benchmark suite, since optimizations for different key types
are likely to be disjoint. ForEach tests have also been split
into separate "Iteration" suites, again with the thought that
iteration performance is likely separable from key type.

As part of adding the new tests, rejiggered the existing tests:
  - Map.set and Set.add (along with their Weak variants) no longer
    include construction costs in their scores
  - Moved key logic into the shared common.js

R=arv@chromium.org, machenbach@chromium.org

Review URL: https://codereview.chromium.org/685753004

Cr-Commit-Position: refs/heads/master@{#24969}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24969 ce2b1a6d-e550-0410-aec6-3dcde31c8c00