Update to latest rust stable

Geelightning now depends on rust-stable rather than rust-nightly.

All nightly features that were in use have either landed in rust stable or their absence can be worked-around with minimal effort.

Tokio, futures, and some other misc crates have had minor updates in order to get them off of alpha versions.

The gitlab CI script now uses the rust toolchain specified in the repo's rust-toolchain file, rather than hard-coding an (old) version.

rust-bitcoin has been updated. We were using a fork of rust bitcoin that contained a fix, but that fix has been merged upstream now.