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 93.89%. Comparing base (8d6ca14) to head (99eaf00).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1457      +/-   ##
==========================================
+ Coverage   93.88%   93.89%   +0.01%     
==========================================
  Files          34       34              
  Lines       15933    15931       -2     
==========================================
  Hits        14958    14958              
+ Misses        975      973       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.