[DNMY] Streamline Dispatch Variable Reference Structure

[pulsipher]

This seeks to address #159. It turns out that dispatching on the index types accounts for a fair portion of the performance limitations: https://discourse.julialang.org/t/unexpected-allocations-with-multiple-dispatch/76988. Hence we have a balance between performance of expression creation (aided by GeneralVariableRefs) and performance of querying values (aided by inhomogeneous datatypes). This PR seeks to find a good compromise.

[codecov[bot]]

Codecov Report

Merging #247 (bda963b) into master (c19d5c7) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #247   +/-   ##
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  Coverage   99.81%   99.81%           
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  Files          33       33           
  Lines        7071     7071           
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  Hits         7058     7058           
  Misses         13       13           

Impacted Files	Coverage Δ
src/general_variables.jl	100.00% <100.00%> (ø)

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