Add packname->stash cache before the check if a packname is a

Add packname->stash cache before the check if a packname is a
filehandle or a package, it works because only packnames
that have been resolved to stashes are added to the cache,
and when a newIO is created we clean the cache.
Results in roughly 1.8 speed increase for class->method()
calls.

p4raw-id: //depot/perl@19133