Minimal versions compat

[mathstuf]

Bump minimum versions so that -Z minimum-versions compilation (mostly) works. Once dtolnay/trybuild#37 has a release, the build-tests crate can use that as a minimum. This patchset was made with removing build-tests from the workspace so that compilation succeeded in the meantime.

I also removed unused dependencies in the meantime (flate2 was particularly useful since it was using tokio-0.1 anyways).

Motivation

-Z minimum-versions is becoming more of a thing and tokio should work with it since it is so prevalent.

Solution

Find oldest versions of crates that failed to work with the flag and set that as the minimum version.

[carllerche]

Thanks! Should this be added to CI? Or should we wait until trybuild has a release?

[mathstuf]

I'd suggest adding a CI run for it once it works. I suppose adding a can-fail variant is fine for now.

Here is a Cirrus chunk for one of my crates. Note that it's not exactly the best since it should probably be testing against the minimum toolchain version as well. In another crate where GitLab-CI is being used, I have separate fetch and build jobs to allow for using nightly to make the lock file and the minimum version to test. I haven't looked into Cirrus enough to know how to structure jobs so that happens yet.

[carllerche]

Unfortunately, there are conflicts now :( could you merge master and then I can merge this PR. 👍

[mathstuf]

Rebased since master removed the tracing dependency which was bumped here.

[mathstuf]

Seems that most of this has been made obsolete by other changes on master.