WIP: rationalize dispatch code

[odow]

This PR is an attempt to understand the MA dispatch code and remove some unneeded functions.

I will split the PR into smaller chunks, but I just wanted to see if it passes CI, and where the coverage issues are.

[codecov]

Codecov Report

Merging #78 (d333f0c) into master (d514a51) will increase coverage by 1.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master      #78      +/-   ##
==========================================
+ Coverage   85.36%   86.37%   +1.01%     
==========================================
  Files          19       19              
  Lines        1353     1314      -39     
==========================================
- Hits         1155     1135      -20     
+ Misses        198      179      -19     

Impacted Files	Coverage Δ
src/dispatch.jl	49.33% <88.88%> (-0.67%)	⬇️
src/Test/generic.jl	24.00% <0.00%> (-8.00%)	⬇️
src/broadcast.jl	74.19% <0.00%> (-0.81%)	⬇️
src/rewrite.jl	89.88% <0.00%> (-0.71%)	⬇️
src/sparse_arrays.jl	85.12% <0.00%> (+3.97%)	⬆️

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[odow]

Closing as stale