refactor: new buffer abstractions in decoders

[westonpace]

Introduce LanceBuffer, copy-on-write version designed for interoperability

between arrow_buffer::Buffer, bytes::Bytes, and Vec (and their mutable counterparts). Use this to remove some copies on the decode path (the value decoder is now truly zero-copy).

Introduce the DataBlock and a concept of "data layouts". Use this to simplify decoding pathways.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 70.00000% with 96 lines in your changes missing coverage. Please review.

Project coverage is 79.38%. Comparing base (a23ba62) to head (7be3f04).

Files	Patch %	Lines
rust/lance-encoding/src/data.rs	59.43%	34 Missing and 9 partials ⚠️
rust/lance-encoding/src/encodings/physical/fsst.rs	0.00%	27 Missing ⚠️
rust/lance-encoding/src/buffer.rs	62.85%	13 Missing ⚠️
...ust/lance-encoding/src/encodings/physical/basic.rs	76.92%	0 Missing and 3 partials ⚠️
...ance-encoding/src/encodings/physical/dictionary.rs	86.36%	1 Missing and 2 partials ⚠️
.../lance-encoding/src/encodings/logical/primitive.rs	86.66%	0 Missing and 2 partials ⚠️
...st/lance-encoding/src/encodings/physical/binary.rs	90.00%	0 Missing and 2 partials ⚠️
...encoding/src/encodings/physical/fixed_size_list.rs	71.42%	0 Missing and 2 partials ⚠️
rust/lance-encoding/src/decoder.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2648      +/-   ##
==========================================
- Coverage   79.38%   79.38%   -0.01%     
==========================================
  Files         221      222       +1     
  Lines       64718    64544     -174     
  Branches    64718    64544     -174     
==========================================
- Hits        51376    51237     -139     
+ Misses      10377    10343      -34     
+ Partials     2965     2964       -1     

Flag	Coverage Δ
unittests	79.38% <70.00%> (-0.01%)	⬇️

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[wjones127]

This looks very nice.. I think the DataBlock seems like a very sensible abstraction 👍

[wjones127]

nit: this could probably be a u8 or something.