fuzz: copy some more files from rust-bitcoin #760

Job	Run time
Docs - stable toolchain	1m 15s
Cross test	1m 43s
Lint - nightly toolchain	1m 45s
Test - nightly toolchain (recent)	5m 45s
Check WASM	30s
Test - stable toolchain (recent)	5m 56s
Test 32-bit version	40s
Bench - nightly toolchain	1m 26s
Test - msrv toolchain (recent)	1m 30s
Docs - nightly toolchain	1m 19s
	21m 49s

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