this-related changes + new String.normalize overload

[Arnavion]

1. Array, TypedArray, Map and Set methods that are specced to return the this-object now return this instead of explicit type.
2. Added an overload for ES6 String.normalize's that has form parameter as union of the allowed string literals.

tests/cases/compiler/implementArrayInterface started failing until I updated it, since it is now an error to implement array methods with old callback that accepted T[]. Breaking change?

jake runtests passes.

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Based on #7664. Since @mhegazy asked for the change to the callbacks to accept this be reverted, and since master split es6.d.ts into multiple files, it was cleaner to just rebase the change onto master than do reverts and merges. To preserve the comments on the diff of that PR I decided to make a new PR.

Note that, as mentioned above, tests/cases/compiler/implementArrayInterface still requires changes to compile (since sort now returns this instead of T[]). I'm not sure I see the value of not taking the callback changes since it's still a breaking change for custom Array<T> implementations.

cc @jods4

[mhegazy]

thanks!