Add symbolic execution #1840

Job	Run time
Build Echidna on ubuntu-20.04	4m 57s
Build Echidna on macos-latest	3m 42s
Build Echidna on windows-latest	19m 10s
Test with solc 0.4.25 on ubuntu-latest	2m 6s
Test with solc 0.5.7 on ubuntu-latest	2m 13s
Test with solc 0.6.12 on ubuntu-latest	2m 17s
Test with solc 0.7.5 on ubuntu-latest	2m 31s
Test with solc 0.4.25 on windows-latest	3m 26s
Test with solc 0.5.7 on windows-latest	3m 21s
Test with solc 0.6.12 on windows-latest	4m 43s
Test with solc 0.7.5 on windows-latest	4m 8s
	52m 34s

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