User/jmaxson/loading fixes

[jmaxson-ms]

When iterating over assemblies and looking for extension types, do not attempt to recreate types that have already been loaded. This can happen if an assembly exists in multiple directories.

The runtime was creating an ILogger which wrote to the console. This was a bug as, in some cases, the caller couldn't override this behavior. The default behavior (runtime) now is to not log if a logger is not specified (Logger.Null). Some methods overrides have been added to allow the caller to specify a logger to use.