Resolve "Speed improvement by compiling the `exec` function"

Closes #4 (closed)

To run the function defined in the Rule class, the exec method is used. I tested this and it is 100x slower than using a regular function. This is because python needs to compile the same function every time. We can also compile the function at time of creating the Rule, therefore making it as fast as regular python code.

\approve @ds

rulelib/rule.py

@@ -44,7 +44,9 @@ class Rule:
 
     def __init__(self, name: str, function: str, inputs: List, outputs: List):
         self.name = name
-        self.function = function
+        # The function is compiled at initialization, so it does not have to be compiled each
+        # time it is executed.
+        self.function = compile(function, "<string>", "exec")
         self.inputs = inputs
         self.outputs = outputs