WIP es7 compliant contains

- [x] es7 polyfil for contains
- [ ] get review on the poly

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- [ ] make enumerable stuff work correctly (ideally without lots of duplication)
- [ ] transition SHIM_ES5 to SHIM_ESNEXT

> I was able to keep a single code-path with near 0 perf difference

I think you'll see a hit in FF/IE vs two code paths, but that can be examined once an implementation lands.