New design for handling unstored values

[mtfishman]

This PR introduces a new design for how unstored values are stored. Instead of a function that generates the unstored values, it is now based around a function unstored(a::AbstractArray) that returns an array that is the same shape as a that when indexed returns the unstored value at that index. By default, that array will be a FillArrays.Zeros, but it can be customized, for example in the case of an array of arrays representing a block array.

Note this is an idea brought up in QuantumKitHub/SparseArrayKit.jl#13.

The motivation is BlockSparseArrays.jl, this design makes it easier to preserve how the zero/unstored values get generated when new sparse arrays are constructed with similar and copy.

[codecov]

Codecov Report

Attention: Patch coverage is 70.76023% with 50 lines in your changes missing coverage. Please review.

Project coverage is 74.78%. Comparing base (1dd8127) to head (01db9ba).

Files with missing lines	Patch %	Lines
src/oneelementarray.jl	63.20%	39 Missing ⚠️
src/sparsearraydok.jl	72.72%	6 Missing ⚠️
src/abstractsparsearrayinterface.jl	70.00%	3 Missing ⚠️
src/abstractsparsearray.jl	93.93%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #65      +/-   ##
==========================================
+ Coverage   74.38%   74.78%   +0.39%     
==========================================
  Files           9        9              
  Lines         648      682      +34     
==========================================
+ Hits          482      510      +28     
- Misses        166      172       +6     

Flag	Coverage Δ
docs	36.02% <21.63%> (+1.66%)	⬆️

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