The jobs should be the same, but Travis may display them differently.

Part of 31859.

Part of 31859.

Since Travis macOS has IPv6 support (and Travis Linux does not), chutney
will now run its IPv6 networks as part of Travis CI.

But since chutney is slow, don't wait for the macOS chutney to finish.
(Travis have fixed the duplicate notification bug in fast_finish. So we
can use fast_finish and allow_failure to finish early. Unfortunately,
allow_failure also means we ignore failures in macOS chutney.)

Also make sure that we have:
* a compile on each platform, with each compiler,
* a check on each platform, and
* a check on each compiler.

Finally, sort builds: allow fail last, macOS first, slowest first.

Closes ticket 30860.
Closes ticket 31859 for 0.2.9.

While merging:
* leave out some redundant jobs and build matrix entries

Part of 31859 for 0.3.5.

Since Rust on macOS is slow, don't wait for the macOS Rust job to finish.
Instead, split rust into slow rust (macOS) and fast rust (Linux). And
allow the build to finish before slow rust finishes.

Also make sure that we have:
* a Rust build on each platform,
* a Rust build with each compiler, and
* a check on all our Rust builds.

Finally, sort builds: allow fail last, macOS first, slowest first.

Closes 31859 for 0.3.5.

Merge TOR_TEST_RNG_SEED in maint-0.4.1 into the coverage line
from bug31859_040.

Merge master's RUST_VERSION="nightly" into the macOS Rust job
from bug31859_041.

Drop master's RUST_VERSION="beta", because we only have two
Rust jobs now.

When we merged TOR_RUST_VERSION from master, the allow_failures rule
did not match any more. Update it to make it match.

Closes 31859 for master.