Improve class loading.
This fixes one bug when dogfooding where the newish fast classloading failed on some types so we crashed.  We now fallback to the old slow version, which works on those wird cases.
This also includes two optimizations.  First, the old tryToLoadTypes method, which was like the old slow classloading, is now gone, so code that used it (including prefetching types during expression generation) can now use the new faster version.  Second, the new fast classloading now extracts and caches the type, which means the next call to getFullyQualifiedType for this type will be faster.  These two changes have a noticeable impact on some benchmarks.