source files.
Normalize the header include statement within the documentation.
Limit `AMpush()` usage within the quickstart example to variable
assignment.
`automerge::AutoCommit::get_at()`.
Add `AMmapRange()` to expose `automerge::AutoCommit::map_range()` and
`automerge::AutoCommit::map_range_at()`.
Add `AMmapItems` for `AMlistRange()`.
Add `AMmapItem` for `AMmapItems`.
