Add symbolic execution #1869

Job	Run time
Build Echidna on ubuntu-20.04	5m 10s
Build Echidna on macos-latest	1m 50s
Build Echidna on windows-latest	25m 33s
Test with solc 0.4.25 on ubuntu-latest	4m 45s
Test with solc 0.5.7 on ubuntu-latest	3m 28s
Test with solc 0.6.12 on ubuntu-latest	3m 20s
Test with solc 0.7.5 on ubuntu-latest	3m 37s
Test with solc 0.4.25 on windows-latest	3m 3s
Test with solc 0.5.7 on windows-latest	3m 31s
Test with solc 0.6.12 on windows-latest	4m 11s
Test with solc 0.7.5 on windows-latest	4m 20s
	1h 2m 48s

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