Various Rope updates and fixes (#3629)
Updates and fixes a few items:
- Fix: Prevents a Rope from containing an empty `Bytes` section
- Perf: Use a `Box<[RopeElem]>` instead of a `Vec<RopeElem>`, reducing
memory useage
- Perf: Implement fast path for `Rope::to_str()` when containing a
single `Shared` element
The first one is the important change. The assumption while iterating
(for streaming, equality checks, etc) is that the only time we can hit
an empty `Bytes` section is when we reach the end of the entire Rope. If
an empty section is hit in the middle of a rope, we can't tell if we've
reached the end or we're in the middle without calling N more times (eg,
what if an empty section followed an empty section, then we'd need to
call 2 more times to see).
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Co-authored-by: Justin Ridgewell <justin@ridgewell.name>
