Add support for optimization mode

[anishnaik]

Optimization mode can be treated as another test case provider, similar to property or assertion mode. This is a great issue for someone who is starting to work with medusa because it touches on medusa's most critical packages, how medusa uses the subscribe/publish model to handle fuzz testing, and how easy it is to create a new testing strategy in medusa.

I would recommend understanding how Echidna's optimization mode works first. At a high level, the optimization mode in Echidna tries to maximize some value x. It probably does this by storing the latest maximum, max, and upon the running of a new call sequence will check whether the new max is greater than max. If so, it overwrites max with the new maximum. Realistically, it doesn't matter what x is, what you care about is the return value of the echidna_opt (medusa_opt) function.

This can be done in medusa as well, please refer to test_case.go, test_case_assertion.go, test_case_assertion_provider.go, test_case_property.go, and test_case_property_provider.go. Similarly, you will probably create a test_case_optimization.go and a test_case_optimization_provider.go.

Here are some key considerations:

1. Support for optimization mode in the config and CLI. Should be disabled by default.
2. Use the property and assertion test case providers as examples on how to build out optimization mode.
3. Create a unit test or two that runs medusa in optimization mode.