buffer: make Buffer binding and prototype setup more straight-forward

buffer: move initialization of buffer prototype into node.js

Instead of exposing setBufferJS in lib/internal/buffer.js after deleting it from the binding and then do the initialization in lib/buffer.js, which results in an implicit dependency on the order in which these modules are loaded.

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

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines