Add symbolic execution #1850

Job	Run time
Build Echidna on ubuntu-20.04	5m 10s
Build Echidna on macos-latest	3m 35s
Build Echidna on windows-latest	8m 53s
Test with solc 0.4.25 on ubuntu-latest	3m 21s
Test with solc 0.5.7 on ubuntu-latest	3m 5s
Test with solc 0.6.12 on ubuntu-latest	2m 49s
Test with solc 0.7.5 on ubuntu-latest	3m 21s
Test with solc 0.4.25 on windows-latest	1m 50s
Test with solc 0.5.7 on windows-latest	1m 47s
Test with solc 0.6.12 on windows-latest	1m 48s
Test with solc 0.7.5 on windows-latest	1m 41s
	37m 20s

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