buffer: allow Uint8Array input to methods

Allow all methods on `buffer` and `Buffer` to take `Uint8Array`
arguments where it makes sense. On the native side, there is
effectively no difference, and as a bonus the `isUint8Array`
check is faster than `instanceof Buffer`.

PR-URL: https://github.com/nodejs/node/pull/10236
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>

test/parallel/test-buffer-concat.js

@@ -35,7 +35,8 @@ function assertWrongList(value) {
     Buffer.concat(value);
   }, function(err) {
     return err instanceof TypeError &&
-           err.message === '"list" argument must be an Array of Buffers';
+           err.message === '"list" argument must be an Array of Buffer ' +
+                           'or Uint8Array instances';
   });
 }
 
@@ -60,3 +61,7 @@ assert.deepStrictEqual(Buffer.concat([empty], 4096), Buffer.alloc(4096));
 assert.deepStrictEqual(
     Buffer.concat([random10], 40),
     Buffer.concat([random10, Buffer.alloc(30)]));
+
+assert.deepStrictEqual(Buffer.concat([new Uint8Array([0x41, 0x42]),
+                                      new Uint8Array([0x43, 0x44])]),
+                       Buffer.from('ABCD'));