Function: eieio-generic-call-primary-only

Call METHOD with ARGS for methods with only :PRIMARY implementations.
ARGS provides the context on which implementation to use.
This should only be called from a generic function.

This method is like `eieio-generic-call', but only
implementations in the :PRIMARY slot are queried. After many
years of use, it appears that over 90% of methods in use
have :PRIMARY implementations only. We can therefore optimize
for this common case to improve performance.