WIP es7 compliant contains

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

@domenic using a faster isNan check I was able to keep a single code-path with negligible perf difference.

I _believe_ `x !== x` is only true for `NaN === NaN`