Add symbolic execution #1905

Job	Run time
Build Echidna on ubuntu-20.04	5m 27s
Build Echidna on macos-13	31m 48s
Build Echidna on windows-latest	25m 21s
Test with solc 0.4.25 on ubuntu-latest	4m 40s
Test with solc 0.5.7 on ubuntu-latest	3m 20s
Test with solc 0.6.12 on ubuntu-latest	3m 12s
Test with solc 0.7.5 on ubuntu-latest	4m 3s
Test with solc 0.4.25 on windows-latest	4m 48s
Test with solc 0.5.7 on windows-latest	4m 17s
Test with solc 0.6.12 on windows-latest	4m 47s
Test with solc 0.7.5 on windows-latest	6m 25s
	1h 38m 8s

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