buffer: implement `iterable` interface

This makes possible to use for..of loop with buffers. Also related keys, values and entries methods are added for feature parity with Uint8Array.

This is basically the code from v8 array iterator implementation minus spec stuff.

Well, I think result object are supposed to be simple mutable data objects (https://people.mozilla.org/~jorendorff/es6-draft.html#sec-createiterresultobject). It is not specified anywhere that custom iterators should work the same way (https://people.mozilla.org/~jorendorff/es6-draft.html#sec-iteratorresult-interface), but I think they should.