Switch cryptography library & implement HSTS.

Fix for more recent bytestring versions.

Otherwise it doesn't know which `strip` function to use...

Integrate new HTTP cache.

And while I'm at it, implement HTTP redirects myself to report correct URIs to the caller & cache.

Add support for cache validation.

This required redesigning the internal APIs & refactoring the code to read better.

Utilize the cache HURL's been writing!

Cache everything for supported status codes.

Write appropriate HTTP responses to the cache!

Implement key-value storage for caching (TODO limit disk usage)

Draft code to check whether to cachen an HTTP response.