Expose share counts in Buffer

[ritchie46]

For polars extension types' dark unsafe magic I need to know how often underlying data is shared. This was possible when we Arced the arrays.

This exposes a method to get the shared count of the Buffer's data.

[codecov]

Codecov Report

Merging #1072 (3b83764) into main (83082fa) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1072      +/-   ##
==========================================
- Coverage   81.02%   81.01%   -0.02%     
==========================================
  Files         366      366              
  Lines       35050    35056       +6     
==========================================
  Hits        28399    28399              
- Misses       6651     6657       +6     

Impacted Files	Coverage Δ
src/buffer/immutable.rs	88.88% <0.00%> (-7.12%)	⬇️

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