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

Missing ToLength will cause it to fail these tests: https://github.com/domenic/Array.prototype.contains/blob/3fb531d074eddf5d23e0a46b57ea48932e1fac69/test/Array.prototype.contains_length-positive.js