Fuzz

Add a `Threshold` type #225

Job	Run time
fuzz (roundtrip_miniscript_str)	6m 20s
fuzz (roundtrip_miniscript_script)	6m 9s
fuzz (parse_descriptor)	5m 58s
fuzz (roundtrip_semantic)	6m 9s
fuzz (parse_descriptor_secret)	7m 11s
fuzz (roundtrip_descriptor)	6m 2s
fuzz (roundtrip_concrete)	6m 9s
fuzz (compile_descriptor)	6m 1s
verify-execution	9s
	50m 8s

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