4.0.0 release

[yurrriq]

Close #94 (and #100).

If possible, I would like to address/include:

• Improve documentation #38 (I think we just need to make sure the docs get published correctly)
• on otp-21+ use uri_string to parse uri #93 merged
• wip: using {active, 100} instead of recv #88 punt to 5.0 or so
• fix request scheme, host and port in req record #110

[yurrriq]

We'll want to document the breaking changes (so far I think it's just the header case sensitivity).

[tsloughter]

Improved edocs to publish to hexdocs would be great to include. I'd commit to writing them but I'm currently struggling to write docs for opentelemetry so don't see myself getting to it very soon.