Rewrite broadcasting and mapping to make it simpler and more general

[mtfishman]

Rewrite broadcasting and mapping to make it simpler and more general. The strategy is to rewrite broadcasting expressions involving KroneckerArrays in terms of an internal Broadcasted-like lazy type called Sum which represents a linear combination of objects (normally arrays but they could themselves be broadcast expressions) with corresponding scalar coefficients.

The idea is that because we want to preserve the KroneckerArray structure, as we construct the broadcast expression we catch operations that are linear and build a Sum expression for that linear expression. Then, we can make use of that linear structure when materializing the broadcast expression.

This also flips the implementation around and implements map in terms of broadcast, which simplifies things a lot.

[codecov]

Codecov Report

Attention: Patch coverage is 73.65591% with 49 lines in your changes missing coverage. Please review.

Project coverage is 79.85%. Comparing base (0b5388f) to head (36f1875).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/linearcombination.jl	58.06%	26 Missing ⚠️
src/kroneckerarray.jl	76.04%	23 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
+ Coverage   78.05%   79.85%   +1.80%     
==========================================
  Files          10       11       +1     
  Lines         679      695      +16     
==========================================
+ Hits          530      555      +25     
+ Misses        149      140       -9     

Flag	Coverage Δ
docs	0.00% <0.00%> (ø)

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