Replace duplexify and friends with readable-stream v4

[vweevers]

Combines #6 and #1. I've forked length-prefixed-stream to make that use readable-stream v4 as well.

Semver-major because the streams exposed here now don't emit duplexify's custom events (prefinish, preend, cork, uncork) although it's unlikely that anyone is using those events (or even many-level itself, which is relatively new).

One thing left to do: test against rave-level (which also depends on readable-stream and failed a test: Level/rave-level#3).

cc @ronag

[mcollina]

lgtm

[ronag]

Woho!

[vweevers]

One thing left to do: test against rave-level (which also depends on readable-stream and failed a test: Level/rave-level#3).

That test failure is unrelated to many-level. See nodejs/readable-stream#477

[vweevers]

I managed to work around that, but then another issue popped up. Don't want to spend more time on that, so I'm just gonna drop support of Node.js 12 in rave-level.

In any case, this PR is good to go.