Put stacks in their own chunks (and cleanup)

[shwestrick]

This patch gives each stack its own chunk, similar to large arrays. Some advantages:

• Stacks can be resized in-place (if the chunk is large enough) without copying any data.
• When a stack needs to be resized larger than its current chunk, we can copy it to a new chunk and immediately reclaim the old chunk.
• During GC, stack chunks can be relocated to the to-space without any copying.
• We should no longer see the strange behavior of stacks "moving around" when they are resized. (Previously, resizing a stack allocated a new stack at the current frontier, which could be deeper than the original location of the stack.)

I did some quick tests and found that these changes are sometimes significant, e.g. in a stack-stress benchmark like fib I saw 20% improvement in space on 70 cores. Elsewhere, I don't think this change affects time or space too much.

To implement this change, I disabled stack shrinking at local GCs. Perhaps this can be re-enabled in the future, but it doesn't seem outrageously important, since the threading in MPL is so well disciplined.