WIP : Implement a `sortedmulti_a` descriptor #304

Job	Run time
fuzz (roundtrip_semantic)	4m 45s
fuzz (parse_descriptor)	4m 38s
fuzz (parse_descriptor_secret)	4m 33s
fuzz (roundtrip_miniscript_script)	4m 39s
fuzz (roundtrip_miniscript_str)	4m 31s
fuzz (roundtrip_descriptor)	4m 48s
fuzz (roundtrip_concrete)	4m 33s
fuzz (compile_descriptor)	4m 33s
verify-execution	11s
	37m 11s

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