Introduces new "mapping point cursors" for the different display map
layers allowing one to map multiple points in increasing order more
efficiently than using the one shot operations.
This is used in the `BlockMap::sync` for `header_and_footer_blocks`
which spends a significant time in sumtree traversal due to repeatedly
transforming points between the different layers. This effectively turns
the complexity of those operations from quadratic in the number of
excerpts to linear, as we only go through the respective sumtrees once
instead of restarting from the start over and over again.
Release Notes:
- Improved performance for editors of large multibuffers with many
different files
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