Save corpus and reproducers continuously (#1167) #1761

Job	Run time
Build Echidna on ubuntu-20.04	3m 55s
Build Echidna on macos-latest	5m 1s
Build Echidna on windows-latest	14m 18s
Test with solc 0.4.25 on ubuntu-latest	3m 28s
Test with solc 0.5.7 on ubuntu-latest	3m 29s
Test with solc 0.6.12 on ubuntu-latest	3m 37s
Test with solc 0.7.5 on ubuntu-latest	4m 8s
Test with solc 0.4.25 on windows-latest	4m 48s
Test with solc 0.5.7 on windows-latest	4m 56s
Test with solc 0.6.12 on windows-latest	6m 10s
Test with solc 0.7.5 on windows-latest	4m 55s
	58m 45s

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