Add a Deferred data type for lazy function call

Add a data type Deferred that is used to describe a function call that is to be performed in the future (at some time when a resolution/call is wanted). The data type should be an Object and have the attributes name - the name of the function, and optional arguments an array with arguments to the function.

The data type is not parameterized. While it would be of interest to have a Deferred[T] where T is the return type - this gets complicated as it is not possible in general to infer what the return type of a variable is because of overloading. We do not have such functionality (yet) in puppet and adding this could be done sometime in the future to ensure that the result of a deferred has a particular data type. This will most likely also mean that an instance of Deferred will need to have that type as a type constraint. The same can however be achieved by making a deferred call to assert_type, and giving a Deferred as an argument.