Add symbolic execution #1891

Job	Run time
Build Echidna on ubuntu-20.04	22m 42s
Build Echidna on macos-13	34m 48s
Build Echidna on windows-latest	25m 44s
Test with solc 0.4.25 on ubuntu-latest	4m 45s
Test with solc 0.5.7 on ubuntu-latest	3m 32s
Test with solc 0.6.12 on ubuntu-latest	3m 50s
Test with solc 0.7.5 on ubuntu-latest	3m 47s
Test with solc 0.4.25 on windows-latest	2m 59s
Test with solc 0.5.7 on windows-latest	3m 37s
Test with solc 0.6.12 on windows-latest	4m 8s
Test with solc 0.7.5 on windows-latest	4m 37s
	1h 54m 29s

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