Tests

feat: add `Hash256` type (#496) #971

Job	Run time
no-std build	1m 30s
Unix tests (ubuntu-latest, stable)	3m 9s
Unix tests (ubuntu-latest, nightly)	2m 49s
WASM tests	7m 7s
Unix tests (macos-11, stable)	4m 24s
Windows tests (stable, x86_64-pc-windows-gnu)	5m 35s
Unix tests (macos-11, nightly)	3m 50s
Windows tests (stable, x86_64-pc-windows-msvc)	8m 56s
Windows tests (nightly, x86_64-pc-windows-gnu)	8m 13s
Windows tests (nightly, x86_64-pc-windows-msvc)	3m 50s
	49m 23s

Provide feedback