Deprecate `Message::from_digest_slice` #80

Job	Run time
Test 32-bit version	25s
Test - nightly toolchain	3m 47s
Test - beta toolchain	30s
Test - stable toolchain	3m 27s
Check for changes to the public API	1m 25s
Cross test	1m 5s
Test - MSRV toolchain	5m 28s
WASM	1m 47s
	17m 54s

Provide feedback