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.

I'm curious -- can iterators reuse results? If so that might be ideal, otherwise for large buffers we might see a *huge* number of throwaway objects generated when iterated with `for... of`.