Parallel garbage collection for SBCL (2023)

- The parallel garbage collector is now part of SBCL, but not yet the default. Enabling it is attractive, though, because it has roughly twice the throughput.

- The SBCL maintainers used the parallel GC as reason to make SBCL's memory management overall more modular. This process is still ongoing, but experimenting with different GC strategies on SBCL has never been easier.

- Another GC is already in the works that has virtually zero pause times. Once this is merged, we can finally bury that old myth that Lisp systems stutter (if it hasn't already been buried by the presence of video games written in Lisp, like Kandria)

The parallel GC is an amazing achievement by Hayley Patton. We SBCL users cannot thank her enough for her outstanding work!