Prune list iterators

[antiochp]

We want to merge #3576 and #3575 before we merge this.
Likely to be some conflicts as I pulled those changes out as separate PRs.

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Lets us do the following -

  let bitmap: Bitmap = prune_list
    .unpruned_leaf_iter(cutoff_pos)
    .collect();

Which is a really convenient way of producing a bitmap representing unpruned leaf positions based on a prune_list.
This in effect "inverts" the prune_list giving the leaves that remain after pruning.

We can take advantage of this for more efficient pruning/compaction.

It is possible to efficiently compute the difference between what was pruned last time and what needs to be pruned this time -

20210224 09:17:13.613 DEBUG grin_store::pmmr - ***** prev_leaf_bitmap: 209732
20210224 09:17:13.616 DEBUG grin_store::pmmr - ***** next_leaf_bitmap: 196498
20210224 09:17:13.616 DEBUG grin_store::pmmr - ***** diff between bitmaps: 15467