Speed up shrinking (#1192) #1797

Job	Run time
Build Echidna on ubuntu-20.04	3m 44s
Build Echidna on macos-latest	2m 57s
Build Echidna on windows-latest	8m 40s
Test with solc 0.4.25 on ubuntu-latest	2m 58s
Test with solc 0.5.7 on ubuntu-latest	3m 45s
Test with solc 0.6.12 on ubuntu-latest	3m 15s
Test with solc 0.7.5 on ubuntu-latest	3m 57s
Test with solc 0.4.25 on windows-latest	4m 7s
Test with solc 0.5.7 on windows-latest	4m 25s
Test with solc 0.6.12 on windows-latest	5m 2s
Test with solc 0.7.5 on windows-latest	4m 46s
	47m 36s

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