Remove rustls-native-roots dependency

[jsha]

There are some issues integrating with trust stores: rustls/rustls-native-certs#16, and also some issues with regards to what gets run in forked processes: kornelski/rust-security-framework#136.

[tgeoghegan]

This breaks the tests on macOS, because /etc/ssl/certs/ca-certificates.crt doesn't exist there. I don't have it on my Fedora 34 install either; instead I have /etc/ssl/certs/ca-bundle.crt. And I doubt that bundle exists on Windows. curl hosts the Mozilla CA bundle at https://curl.se/ca/cacert.pem, so perhaps we could at least make CI tests behave reliably by making the makefile fetch the cert bundle before running crustls-demo.

[tgeoghegan]

Or an alternative that wouldn't depend on httpbin.org's certain chain being trusted by the CA bundle currently hosted by curl.se would be to fetch the cert chain for httpbin.org and then pass that to crustls-demo as a trust store, since the point of crustls's tests is to exercise making connections over HTTPS rather than certificate trust evaluation. Continuing on that line of thought, the most robust way forward might be to have the tests spin up a local HTTPS server with a self-signed certificate, which would remove all external dependencies and even make it possible to run crustls tests without reaching the internet.

[jsha]

Merged latest main, which includes a switch to talking to a local server and using a locally trusted root.

Addressed changes.