Map: get, set, has, delete, clear
Set: add, has, delete, clear

All except clear are implemented as calls into collection.js.

Note that some of these shadow methods of v8::Object. It's unclear
how confusing that's going to be: on the one hand, it seems likely
that most operations you would want to do on a Map or Set are these.
On the other, generic code could get confused if it somehow gets
ahold of a variable that happens to be C++-typed as a v8::Map or v8::Set.

BUG=v8:3340
LOG=y

Review URL: https://codereview.chromium.org/1204623002

Cr-Commit-Position: refs/heads/master@{#29237}