Initialize environment with primitives
Modify eval & apply to use functions in the environment
Change 'NumArgs Integer' to 'NumArgs Int' in data LispError
Expand apply to work on non-primitive functions
Expand eval to allow function definitions
Simplify showVal on Funcs & remove extraneous quotes
Simplify showError for NotFunction
Throw error when applying a non-function
Simplify showError for UnboundVar