Examine ProxyBuilder::referencedTypes startup cost

Analysing profiles of applications doing annotation processing during startup show that ProxyBuilder::referencedTypes can take a disproportionate amount of time. Mainly this is due to depending on numerous stream classes and setting up a number of lambdas, but also minor things like cloning the arrays of parameter and exception types play a part.

Desugaring and optimizing the code to calculate referencedTypes should be considered.