Implement envOr(...) cheatcodes

[RaoulSchaffranek]

This PR adds envOr(...) cheatcodes.

There are some design choices to consider:

• Should env variables be read just in time, when the cheatcode call happens, or should env variables be read during initialization? For Simbolik, reading environment variables on demand doesn't make sense because it's running on a server and not on the developer's machine. I think there should be a way to populate environment variables with a new CLI argument, such as --set-env-var name value. These variables are then stored in a new subconfiguration <envVars> and always read from there.
• What should be the default behavior if a variable is undefined? Particularly for symbolic execution? Should the call return the default value or some fresh variable? Fresh variables sound like the safest option, because they're an overapproximation, but could also lead to more branching.