Specialize norm-related functions for Diagonal

[jishnub]

For banded matrices such as Diagonal, we may skip the non-stored elements in computing the norm, as the contribution of these would be zero.

This improves performance.
On master

julia> D1 = Diagonal(rand(1000));

julia> D2 = Diagonal(rand(1000));

julia> @btime norm($D1);
  2.668 ms (0 allocations: 0 bytes)

julia> @btime isapprox($D1, $D2);
  8.007 ms (3 allocations: 7.88 KiB)

vs this PR

julia> @btime norm($D1);
  247.196 ns (0 allocations: 0 bytes)

julia> @btime isapprox($D1, $D2);
  1.696 μs (0 allocations: 0 bytes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.90%. Comparing base (7b21cab) to head (73aee00).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1457      +/-   ##
==========================================
+ Coverage   93.89%   93.90%   +0.01%     
==========================================
  Files          34       34              
  Lines       15926    15934       +8     
==========================================
+ Hits        14953    14963      +10     
+ Misses        973      971       -2     

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