Update to 2021 edition, add ci + clippy fixes #1

Job	Run time
Clippy	37s
Build docs (stable, macos-latest)	45s
Build docs (stable, ubuntu-latest)	15s
Build docs (stable, windows-latest)	1m 24s
Rustfmt	3s
Check Powerset of Features (stable, macos-latest)	1m 5s
Check Powerset of Features (stable, ubuntu-latest)	25s
Test (stable, macos-latest)	21s
Check Powerset of Features (stable, windows-latest)	1m 22s
Test (stable, ubuntu-latest)	1m 17s
Check Powerset of Features (beta, macos-latest)	1m 3s
Test (stable, windows-latest)	1m 22s
Check Powerset of Features (beta, ubuntu-latest)	32s
Check Powerset of Features (beta, windows-latest)	1m 53s
Check Powerset of Features (nightly, macos-latest)	1m 0s
Check Powerset of Features (nightly, ubuntu-latest)	37s
Check Powerset of Features (nightly, windows-latest)	1m 28s
Result (CI)	0s
	15m 29s

Provide feedback