* use dyn Trait syntax where appropriate
recent rust nightly started warning that not using `dyn` was
deprecated. This requires MSRV 1.27.0+.
* rustfmt fallout from dyn additions
* stop explicit allow of rust_2018_idioms
* more dyn Trait syntax
* drop tokio-macros from 0.1.x workspace
Since tokio-macros specifies an edition=2018, we would otherwise
require MSRV 1.31.0 to build/test it. And tokio-macros isn't used with
tokio 0.1.x.
* reactor: narrow tokio-io-pool dev dep to 0.1.4
Since 0.1.5-6 is now a edition=2018 crate, which has effective MSRV
1.31.0.
* narrow tempfile dev-dep to avoid MSRV bump
tempfile 3.1.0 pulls in rand 0.7.0 and is MSRV 1.32.0
* narrow flate2 dev-dep to avoid MSRV bump
flate2 1.0.10-11 have MSRV 1.34.0.
github refs: alexcrichton/flate2-rs#207
* fs: drop deprecated tempdir crate use in tests
In particular because it pulls in old rand duplicates. Replace use
with tempfile::tempdir() which has been available since tempfile
3.0.0.
backport-of: #1312
* increase CI MSRV to 1.28.0
David Kellum