Clean up hashes and related types ahead of `bitcoin_hashes` 1.0 #705

Job	Run time
Bench - nightly toolchain	2m 2s
Docs - stable toolchain	1m 32s
Check WASM	31s
Test - nightly toolchain (recent)	5m 44s
Docs - nightly toolchain	1m 49s
Test - msrv toolchain (recent)	1m 41s
Lint - nightly toolchain	2m 10s
Test - stable toolchain (recent)	5m 36s
Cross test	1m 49s
Test 32-bit version	41s
	23m 35s

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