Implements concurrent `Smt::compute_mutations` #284

Job	Run time
test stable on ubuntu with default	3m 59s
test stable on ubuntu with smt-hashmaps	4m 9s
test stable on ubuntu with no-std	3m 27s
test nightly on ubuntu with default	4m 13s
test nightly on ubuntu with smt-hashmaps	3m 51s
test nightly on ubuntu with no-std	4m 1s
	23m 40s

Provide feedback