Add symbolic execution #1854

Job	Run time
Build Echidna on ubuntu-20.04	16m 51s
Build Echidna on macos-latest	29m 6s
Build Echidna on windows-latest	9m 24s
Test with solc 0.4.25 on ubuntu-latest	3m 25s
Test with solc 0.5.7 on ubuntu-latest	2m 49s
Test with solc 0.6.12 on ubuntu-latest	2m 45s
Test with solc 0.7.5 on ubuntu-latest	3m 34s
Test with solc 0.4.25 on windows-latest	4m 9s
Test with solc 0.5.7 on windows-latest	3m 48s
Test with solc 0.6.12 on windows-latest	5m 13s
Test with solc 0.7.5 on windows-latest	4m 48s
	1h 25m 52s

Provide feedback