Deprecate `Message::from_digest_slice` #79

Job	Run time
Cross test	1m 2s
Test - beta toolchain	30s
Check for changes to the public API	1m 31s
Test 32-bit version	24s
Test - MSRV toolchain	5m 29s
Test - nightly toolchain	2m 2s
WASM	1m 41s
Test - stable toolchain	1m 46s
	14m 25s

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