[roll] Roll fuchsia [kernel] Implement SpscBuffer reads and writes

This CL finishes up the implementation of the SpscBuffer by adding
support for reads and writes.

The Read function is relatively straightforward in its behavior, but
notably requires the caller to pass in a CopyOutFn. This argument
allows the usage of different copy functions in different environments.
Concretely, this allows use memcpy in the tests, and will allow us to
use usercopy in the kernel.

The Write operation is a little more complex, since it must be
transactional in nature. To support this, logical Writes are split into
three separate methods:
1. A Reserve method, which verifies that a slot of the requested
   size exists in the ring buffer and returns that slot as a
   Reservation object.
2. A Write method, called on the Reservation object, which actually
   writes data into the pre-reserved slot.
3. A Commit method, called on the Reservation object, which moves the
   write pointer forward and thus makes the write visible to reads.

Original-Bug: 404539312
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1235160
Original-Revision: 4d890f51ebe9eb4157cb04f7a30172555233239e
GitOrigin-RevId: 8eb7723b5bb596a25adef5b6ef864aba9860611e
Change-Id: Iba0a26b374b2d143f287b0deb3a3a826b3985ade