buffer: move Buffer prototype wiring into internal/buffer.js

Instead of exposing the Buffer prototype methods through an
object in `internal/buffer.js` and then iterating over it
to put the methods on the prototype, create a function
in `internal/buffer.js` to do this.

Also moves the creaton of the `FastBuffer` class into
`internal/buffer.js` and expose it directly instead of
writing it onto that module later.

PR-URL: https://github.com/nodejs/node/pull/25292


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>