Band indexing for structured matrices with Fill bands

[jishnub]

After this, the following works:

julia> D = Diagonal(Ones(4));

julia> D[Band(0)]
4-element Fill{Float64}, with entries equal to 1.0

julia> D[Band(-1)]
3-element Fill{Float64}, with entries equal to 0.0

This doesn't work on master, as Zeros can't be converted to Ones. The case with Fill bands is special-cased in this PR, although a more general would be ideal in the longer term.

This also adds band indexing tests for infinite structured matrices.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.03 ⚠️

Comparison is base (3cc551e) 89.39% compared to head (310b1aa) 89.36%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #378      +/-   ##
==========================================
- Coverage   89.39%   89.36%   -0.03%     
==========================================
  Files          24       24              
  Lines        3479     3480       +1     
==========================================
  Hits         3110     3110              
- Misses        369      370       +1     

Impacted Files	Coverage Δ
src/interfaceimpl.jl	97.95% <100.00%> (-2.05%)	⬇️

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