Validate config to avoid VM errors

[0xalpharush]

If a config file has large values, it causes an error that is only evident when running --format text e.g. echidna: VM attempted an illegal operation: IllegalOverflow Ex:

propMaxGas: 0xfffffffffffffffffffffffffffff
testMaxGas: 0xfffffffffffffffffffffffffffff
codeSize: 0xfffffffffffffffffffffffffffff

Add data validation to config parsing

[gustavo-grieco]

Please test #792. Note that I cannot reproduce this issue with latest echidna with small contract (perhaps it can be triggered with large ones)

[arcz]

This is fixed https://github.com/crytic/echidna/pull/935.