This was tested using cargo generate-lockfile -Zminimal-versions.
The following indirect dependencies also have minimal version
dependencies. For at least num, rustc-serialize and rand this is
necessary to compile on rustc versions that are not older than 1.0.
* num = "0.1.27"
* rustc-serialize = "0.3.20"
* termcolor = "1.0.4"
* libudev-sys = "0.1.1"
* rand = "0.3.14"
* ab_glyph = "0.2.7
Based on https://github.com/bevyengine/bevy/pull/2455
# Objective
Reduce compilation time
# Solution
Remove unused dependencies. While this PR doesn't remove any crates from `Cargo.lock`, it may unlock more build parallelism.
* Switch from the deprecated `multitask` crate to `async-executor`
* async-executor appears to be essentially multitask 0.3
* use block_on in futures_lite instead of pollster because futures_lite is already in the dependency list of async-executor
