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>

lib/buffer.js

@@ -36,21 +36,7 @@ const {
   swap64: _swap64,
   kMaxLength,
   kStringMaxLength,
-  zeroFill: bindingZeroFill,
-
-  // Additional Buffer methods
-  asciiSlice,
-  base64Slice,
-  latin1Slice,
-  hexSlice,
-  ucs2Slice,
-  utf8Slice,
-  asciiWrite,
-  base64Write,
-  latin1Write,
-  hexWrite,
-  ucs2Write,
-  utf8Write
+  zeroFill: bindingZeroFill
 } = internalBinding('buffer');
 const {
   getOwnNonIndexProperties,
@@ -87,30 +73,14 @@ const {
 } = require('internal/errors').codes;
 const { validateString } = require('internal/validators');
 
-const internalBuffer = require('internal/buffer');
+const {
+  FastBuffer,
+  addBufferPrototypeMethods
+} = require('internal/buffer');
 
-class FastBuffer extends Uint8Array {}
 FastBuffer.prototype.constructor = Buffer;
-internalBuffer.FastBuffer = FastBuffer;
-
 Buffer.prototype = FastBuffer.prototype;
-
-for (const [name, method] of Object.entries(internalBuffer.readWrites)) {
-  Buffer.prototype[name] = method;
-}
-
-Buffer.prototype.asciiSlice = asciiSlice;
-Buffer.prototype.base64Slice = base64Slice;
-Buffer.prototype.latin1Slice = latin1Slice;
-Buffer.prototype.hexSlice = hexSlice;
-Buffer.prototype.ucs2Slice = ucs2Slice;
-Buffer.prototype.utf8Slice = utf8Slice;
-Buffer.prototype.asciiWrite = asciiWrite;
-Buffer.prototype.base64Write = base64Write;
-Buffer.prototype.latin1Write = latin1Write;
-Buffer.prototype.hexWrite = hexWrite;
-Buffer.prototype.ucs2Write = ucs2Write;
-Buffer.prototype.utf8Write = utf8Write;
+addBufferPrototypeMethods(Buffer.prototype);
 
 const constants = Object.defineProperties({}, {
   MAX_LENGTH: {