* Split `SegmentEntry`
Add `ImmutableSegmentEntry` for operations that can be applied to
immutable segments, and make the `SegmentEntry` as it subtrait.
* Rename to `NonAppendableSegmentEntry`
It differs semantically from `ImmutableSegmentEntry` by allowing point
deletion. Move point deletion to the trait too.
* Fix docstring
* use NonAppendableSegmentEntry where possible
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Co-authored-by: Andrey Vasnetsov <andrey@vasnetsov.com>