Implement `TreeLike` for `policy::Concrete` #193

Job	Run time
fuzz (roundtrip_miniscript_str)	6m 57s
fuzz (roundtrip_miniscript_script)	5m 58s
fuzz (parse_descriptor)	5m 34s
fuzz (roundtrip_semantic)	6m 36s
fuzz (parse_descriptor_secret)	5m 28s
fuzz (roundtrip_descriptor)	5m 45s
fuzz (roundtrip_concrete)	6m 41s
fuzz (compile_descriptor)	5m 42s
verify-execution	5s
	48m 46s

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