Add symbolic execution #1879

Job	Run time
Build Echidna on ubuntu-20.04	5m 13s
Build Echidna on macos-latest	3m 43s
Build Echidna on windows-latest	8m 59s
Test with solc 0.4.25 on ubuntu-latest	4m 48s
Test with solc 0.5.7 on ubuntu-latest	3m 29s
Test with solc 0.6.12 on ubuntu-latest	3m 17s
Test with solc 0.7.5 on ubuntu-latest	3m 44s
Test with solc 0.4.25 on windows-latest	3m 17s
Test with solc 0.5.7 on windows-latest	3m 32s
Test with solc 0.6.12 on windows-latest	4m 13s
Test with solc 0.7.5 on windows-latest	4m 30s
	48m 45s

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