simulator: test the simulator via manual mutation injection

[alpaylan]

This PR injects bugs as comments in several places throughout Limbo, which will be used by Marauders Mutation Injection Tool to switch between different variants and test the simulator.

Note: This PR would ideally merge into base:mutations in order to prevent pollution caused by the variants in the main code.

[alpaylan]

We can start testing the simulator with this change;

The mutation injection tool can be installed with the following command.

cargo install marauders

The shallow optimizer bug can be tested with the following command;

marauders test --expr encode_true_as_false --command "cargo run --bin limbo_sim -- --maximum-size=200 --minimum-size=100"

The marauders test command takes a mutation expression, which defines which mutations should be run, and a command, which should return a non-zero exit code on failure. The details of the mutation expressions are briefly explained on the mutation tool repository.