Clean up the `MiniscriptKey` and `ToPublicKey` traits #386

Job	Run time
fuzz (roundtrip_semantic)	2m 20s
fuzz (parse_descriptor)	2m 20s
fuzz (parse_descriptor_secret)	2m 22s
fuzz (roundtrip_miniscript_script)	2m 25s
fuzz (roundtrip_miniscript_str)	2m 31s
fuzz (roundtrip_descriptor)	2m 19s
fuzz (roundtrip_concrete)	2m 23s
fuzz (compile_descriptor)	2m 29s
verify-execution	7s
	19m 16s

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