buffer: backport new buffer constructor APIs to v4.x

This backports the new `Buffer.alloc()`, `Buffer.allocUnsafe()`,
`Buffer.from()`, and `Buffer.allocUnsafeSlow()` APIs for v4.

Some backported tests are disabled, but those are not related to the
new API.

Note that `Buffer.from(arrayBuffer[, byteOffset [, length]])` is not
supported in v4.x, only `Buffer.from(arrayBuffer)` is.

Refs: #4682
Refs: #5833
Refs: #7475
PR-URL: #7562
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Nikolai Vavilov <vvnicholas@gmail.com>