base: Re-implement CircleBuf without using CircularQueue.

base: Re-implement CircleBuf without using CircularQueue.

CircularQueue provides iterators which make it easier for users to
interact with it and helps abstract its internal state, but at the same
time it prevents standard algorithms like std::copy from recognizing
opportunities to use bulk copies to speed up execution. It also hides
the seams when wrapping around the buffer happens which std::copy
wouldn't know how to handle.

CircleBuf seems to be intended as a simpler type which doesn't hold
complex entries like the CircularQueue does, and instead just acts as a
wrap around buffer, like the name suggests. This change reimplements it
to not use CircularQueue, and to instead use an underlying vector.

The way internal indexing is handled CircularQueue was simplified
recently, and using the same scheme here means that this code is
actually not much more verbose than it was before. It also intrinsically
handles indexing and bulk accesses, and uses std::copy_n in a way that
lets it recognize and take advantage of contiguous storage and bulk
copies.

Change-Id: I78e7cfe174c52f60c95c81e5cd3d71c6052d4d41
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38896
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>