Compleating assignment for interpreter, modified grammars, fixed tests

lib/miniFun/Types.mli

@@ -53,9 +53,18 @@ type memory = {
   assignments: permittedValues VariableMap.t
 }
 
-type error = [
+
+type base_error = [
     `AbsentAssignment of string
   | `WrongType of string
-  | `DivisionByZero of string
+]
+
+type typechecking_error = [
+  | base_error
   | `WrongTypeSpecification of string
 ]
+
+type error = [
+  | base_error
+  | `DivisionByZero of string
+]