sim,mem,dev: Merge eventq_impl.hh into eventq.hh.

sim,mem,dev: Merge eventq_impl.hh into eventq.hh.

Having some methods (which are supposed to be inline) defined in another
file which is only included sometimes creates a lot of opportunities for
errors. They no longer need to be separate, so merge them together.

Change-Id: I5846e55f53f59b9c2081680a6441659265a765f7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29409
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>