refactor: make lambda expression types immutable

- Change Abstraction, Application, and Variable to use private fields with getter methods
- Return value types instead of pointers from constructors
- Update all type switches to match value types instead of pointer types
- Remove pointer equality optimizations (not applicable with immutable values)
- Return empty set instead of nil from GetFreeVariables default case

tests/church_6^6.test

@@ -1,8 +0,0 @@
-0 := \f.\x.x
-inc n := \f x.(f (n f x))
-exp n m := (m n)
-print n := (n F X)
-
-N := (inc (inc (inc (inc (inc (inc 0))))))
-
-(print (exp N N))