Backport to v1.x

Basically everything we could backport to v1.x, including some build & perf improvements, I'm not too sure what we all want to take.

Commits have been slightly re-arranged in some cases to group related commits together.

I guess we should discuss what we do / don't want to take? I'm willing to do the grunt work of any metadata.

cc @rvagg @nodejs/lts (need to make that group soon..) maybe also cc @jasnell, he may have opinions?

Note: don't try to view the changes here, just don't.