merge FastFieldCodecReader wit FastFieldDataAccess

[PSeitz]

• num_vals to FastFieldCodecReader
• split open_from_bytes to own trait
• rename get_u64 to get_val
• merge FastFieldCodecReader and FastFieldDataAccess traits

[codecov-commenter]

Codecov Report

Merging #1485 (50bbec9) into main (17093e8) will decrease coverage by 0.01%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##             main    #1485      +/-   ##
==========================================
- Coverage   94.09%   94.07%   -0.02%     
==========================================
  Files         239      239              
  Lines       44833    44863      +30     
==========================================
+ Hits        42184    42207      +23     
- Misses       2649     2656       +7     

Impacted Files	Coverage Δ
src/fastfield/writer.rs	91.18% <ø> (ø)
src/fastfield/gcd.rs	98.82% <66.66%> (-1.18%)	⬇️
fastfield_codecs/src/lib.rs	95.62% <76.92%> (-1.49%)	⬇️
fastfield_codecs/src/bitpacked.rs	100.00% <100.00%> (ø)
fastfield_codecs/src/blockwise_linear.rs	100.00% <100.00%> (ø)
fastfield_codecs/src/linear.rs	99.57% <100.00%> (+<0.01%)	⬆️
src/fastfield/mod.rs	94.76% <100.00%> (ø)
src/fastfield/reader.rs	87.82% <100.00%> (-0.65%)	⬇️
src/fastfield/serializer/mod.rs	88.60% <100.00%> (ø)
src/indexer/merger.rs	98.94% <100.00%> (ø)
... and 5 more

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[fulmicoton]

I'd put that in the XXXXSerializers and rename the trait XXXXCodec, and add an associated trait over the reader.

The XXXXSerializer and the XXXXDeserializer are not really independent.

[PSeitz]

I think don't fully understand. I split the Deserializer from the FastFieldDataAccess, because e.g. in the merge case we provide access without deserializing.