Deprecate `Message::from_digest_slice` #1333

Job	Run time
Test - stable toolchain	3m 32s
Test - beta toolchain	30s
Test - nightly toolchain	3m 46s
Check for changes to the public API	1m 24s
Test 32-bit version	31s
Test - MSRV toolchain	5m 30s
WASM	1m 44s
Cross test	1m 7s
	18m 4s

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