Provide re-usable shared `MutableAntichain`

[frankmcsherry]

Differential dataflow trace handles have shared state that is roughly multiple "holds" on a mutable antichain, where the antichain itself reflects the lower bound of all expressed holds, and it advances as the various holds advance and are dropped. That pattern is useful outside of DD, and could / should be extracted and made available for others to use.

cc: @mjibson