Bump dependencies, set rust-version, and some more maintenance work

[udoprog]

Reason for this change is that I noticed that the irc crate was the sole cause in my dependency graph for pulling in some old dependencies (parking_lot among others). Some dependencies had CVEs specified for versions which were inside of their minimal ranges. So I went ahead and ran a cargo upgrade --workspace here.

I used a tool which tried to build and detect the MSRV and updated rust-version. It also sorted the fields, as they are specified in the Cargo documentation. Hope this i OK, otherwise I'll pull it back.

Since features need to be additive, I've made it so that if both tls-rust and tls-native are activated, only tls-rust will be used. If you want this flipped, please tell me.

Finally I've added a Github workflow file, which performs most checks (including testing for MSRV). But I haven't ported any integration tests yet. So if you want to keep running them on Travis that is OK by me. You can see the run here.

[udoprog]

Note the odd make_client_auth! macro, that one is necessary because the builder produced by calling with_custom_certificate_verifier vs. with_root_certificates have different return signatures, but still supports the same sequence of methods to configure it.

[aatxe]

Hm, sorry about the delay on this, but looks good. Appreciate the changes a lot, especially the GH Actions setup now that Travis is dead.

[udoprog]

Appreciate it!