Specialize isbanded for OneElement

[jishnub]

This allows O(1) checks for the bandedness of a OneElement.

julia> A = OneElement(3, (20,20), (20,20));

julia> @btime isdiag($A);
  542.590 ns (0 allocations: 0 bytes) # master
  3.382 ns (0 allocations: 0 bytes) # PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.90%. Comparing base (4bfbeec) to head (6902fcc).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #333   +/-   ##
=======================================
  Coverage   99.90%   99.90%           
=======================================
  Files           8        8           
  Lines        1086     1095    +9     
=======================================
+ Hits         1085     1094    +9     
  Misses          1        1           

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[longemen3000]

seems a nice addition to have. i dont see any problems with it at first glance.