show for BandedMatrix may print the constructor

[jishnub]

After this PR

julia> B = BandedMatrix(-1=>1:4, 0=>1:5, 1=>1:4);

julia> show(B)
BandedMatrix(-1 => [1, 2, 3, 4], 0 => [1, 2, 3, 4, 5], 1 => [1, 2, 3, 4])

vs on master

julia> show(B)
[1 1 0 0 0; 1 2 2 0 0; 0 2 3 3 0; 0 0 3 4 4; 0 0 0 4 5]

The new display format is more informative, sparse and a valid constructor as well.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (4f24c9d) 89.51% compared to head (ab18601) 89.54%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #404      +/-   ##
==========================================
+ Coverage   89.51%   89.54%   +0.02%     
==========================================
  Files          25       25              
  Lines        3529     3538       +9     
==========================================
+ Hits         3159     3168       +9     
  Misses        370      370              

Files	Coverage Δ
src/banded/BandedMatrix.jl	95.20% <100.00%> (+0.09%)	⬆️

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