Implement evaluation cache

Created by: dktcoding

As of now, every call to Expression#evaluate() reevaluates the whole expression. This isn't a problem in and of itself, but it's a common error when using the library and creates an annoying performance hit.

• Find all scenarios where this could fail
• If an expression contains non-deterministic results, the value should not be cached
• Cache should be cleaned if variable changes
• What happens when using setVariables() with maps?
• Create tests for expression evaluate