Add symbolic execution #1890

Job	Run time
Build Echidna on ubuntu-20.04	21m 51s
Build Echidna on macos-13	28m 36s
Build Echidna on windows-latest	26m 14s
Test with solc 0.4.25 on ubuntu-latest	4m 40s
Test with solc 0.5.7 on ubuntu-latest	3m 24s
Test with solc 0.6.12 on ubuntu-latest	3m 31s
Test with solc 0.7.5 on ubuntu-latest	3m 55s
Test with solc 0.4.25 on windows-latest	3m 8s
Test with solc 0.5.7 on windows-latest	3m 35s
Test with solc 0.6.12 on windows-latest	4m 12s
Test with solc 0.7.5 on windows-latest	3m 31s
	1h 46m 37s

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