refactor: use list encoder / scheduler directly instead of replicating behavior in map encoder / scheduler

[westonpace]

This is a minor simplification of the recently added map support. It does not add any functionality, only simplifies the code slightly. The current implementation creates a StructuralMapEncoder which mimics the StructuralListEncoder. This change instead wraps the StructuralListEncoder (composition instead of duplication). As a result we can get rid of the StructuralMapScheduler. The decoder then simply casts from a list array to a map array.

Since the list encoder is so simple this isn't really saving us a ton of work. However, if we choose to experiment or add new complicated features to the list encoder in the future then this will help us avoid doing the work twice or missing the changes in the map encoder.

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[westonpace]

CC @xloya do you mind reviewing? I don't feel super strongly about this so if you prefer the old approach that is ok too. Still, I think if we can avoid the map encoder having its own logic that will be helpful.

[codecov[bot]]

Codecov Report

:x: Patch coverage is 84.48276% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-encoding/src/encodings/logical/map.rs	84.44%	5 Missing and 2 partials :warning:
rust/lance-encoding/src/decoder.rs	85.71%	1 Missing :warning:
...ust/lance-encoding/src/encodings/logical/struct.rs	83.33%	0 Missing and 1 partial :warning:

:loudspeaker: Thoughts on this report? Let us know!

[xloya]

I think this optimization is great! There's only one issue regarding the list field naming.