add mpsc example

[matklad]

A twist on asymmetric, with two changes:

• there are many producer threads, not one
• the queue is statically sized, consumer applies backpressure to producers.

This program does O(N) allocations, but has only O(1) peak memory usage.

At the time of writing:

• smp allocator requires O(N) peak memory
• debug and c (on MacOS) need O(1), as expected:

λ ps x -o time,rss,command | grep 'mpsc smp'
  0:41.16  44752 ./zig-out/bin/mpsc smp
λ ps x -o time,rss,command | grep 'mpsc smp'
  0:53.26  51472 ./zig-out/bin/mpsc smp
λ ps x -o time,rss,command | grep 'mpsc smp'
  1:09.11  71344 ./zig-out/bin/mpsc smp

λ ps x -o time,rss,command | grep 'mpsc c'
  0:19.76   1616 ./zig-out/bin/mpsc c
λ ps x -o time,rss,command | grep 'mpsc c'
  0:50.78   1616 ./zig-out/bin/mpsc c
λ ps x -o time,rss,command | grep 'mpsc c'
  0:59.46   1616 ./zig-out/bin/mpsc c
λ ps x -o time,rss,command | grep 'mpsc c'
  1:04.80   1728 ./zig-out/bin/mpsc c

[andrewrk]

Thanks for taking the time to discover and send this, nice find!