Support new Decimal256 type

[quinnj]

No description provided.

[codecov]

Codecov Report

Merging #79 (a95d3d0) into main (a4c4087) will increase coverage by 0.03%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##             main      #79      +/-   ##
==========================================
+ Coverage   83.48%   83.52%   +0.03%     
==========================================
  Files          23       23              
  Lines        2658     2664       +6     
==========================================
+ Hits         2219     2225       +6     
  Misses        439      439              

Impacted Files	Coverage Δ
src/Arrow.jl	52.38% <ø> (ø)
src/eltypes.jl	82.62% <85.71%> (+0.07%)	⬆️
src/metadata/Schema.jl	80.70% <85.71%> (+0.28%)	⬆️

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