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 <[email protected]> Reviewed-By: James M Snell <[email protected]> Reviewed-By: Anna Henningsen <[email protected]> Reviewed-By: Nikolai Vavilov <[email protected]>

Author: ChALkeR