Improve how type mismatches are asserted and reported

Type assertion and reporting could be greatly improved and also made consistent by fixing three minor things.

1. The TypeAsserter methods takes a subject parameter that explains in what context the assertion takes place. It is often desirable to create this subject string from things in that context (i.e. format a string). Formatting strings that may or may not be used is however not a good practice since the formatting itself is resource consuming. Deferred formatting can be enabled by a) allowing an Array argument where the first element is a format string and the remaining elements are parameters to that format string, and b) allowing a block to be passed in the calls to the TypeAsserter. The block is called with the subject parameter as it's argument and returns a new subject.
2. The TypeAsserter currently creates it's own "type mismatch" based on the subject and the compared types. It should instead utilize the TypeMismatchDescriber
3. The Puppet function assert_type has it's own logic for asserting and printing errors. It should use the TypeAsserter.