Replace Buffer constructor with alloc/from following Node.js

[grgustaf]

Node.js now recommends v6.9.2 "for most users," which is much newer than what we originally based our code on. We should try to update our APIs to track the changes and drop deprecated functions.

[grgustaf]

These days the stable version is v8.9.4. I'm going to change the summary to make this more specific to moving away from the deprecated Buffer constructor and use alloc/from instead.