Make continuous fuzzing an option #1821

Job	Run time
Build Echidna on ubuntu-20.04	4m 44s
Build Echidna on macos-latest	4m 32s
Build Echidna on windows-latest	9m 59s
Test with solc 0.4.25 on ubuntu-latest	2m 24s
Test with solc 0.5.7 on ubuntu-latest	2m 39s
Test with solc 0.6.12 on ubuntu-latest	2m 25s
Test with solc 0.7.5 on ubuntu-latest	6h 0m 14s
Test with solc 0.4.25 on windows-latest	3m 23s
Test with solc 0.5.7 on windows-latest	3m 49s
Test with solc 0.6.12 on windows-latest	4m 27s
Test with solc 0.7.5 on windows-latest	6h 0m 9s
	12h 38m 45s

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