buffer: backport new buffer constructor APIs

This backports the new `Buffer.alloc()`, `Buffer.allocUnsafe()`,
and `Buffer.from()` APIs for v5.

Also included in this backport is the change that allows fill('')
to zero-fill (as opposed to doing nothing) and the additional
`byteOffset` and `length` arguments for `Buffer(arrayBuffer)` and
`Buffer.from(arrayBuffer)`.

This backport includes the new test cases.

This backport *does not* update all of the internal uses of the
existing `Buffer()` constructor.

This backport also *does not* include the soft deprecation of the
existing `Buffer()` constructor.