Add conversion matrix and `Convert{Once}` traits #337

Job	Run time
Quick check (beta)	1m 59s
Compile and test MSRV (ubuntu-latest)	8m 47s
Quick check (stable)	1m 58s
integration tests (ubuntu-latest)	27s
Quick check (nightly)	2m 0s
Compile and test MSRV (macos-latest)	5m 56s
integration tests (macos-latest)	24s
Compile and test MSRV (windows-latest)	12m 54s
Test #[no_std]	19s
integration tests (windows-latest)	2m 8s
Documentation	31s
Miri tests	4m 0s
ci	1s
	41m 24s

Provide feedback