fix array fast path on matrices, do N-tensor generation, and fast sparse functions

[ChrisRackauckas]

Fixes #231

[codecov]

Codecov Report

Merging #233 into master will decrease coverage by 1.5%.
The diff coverage is 90.9%.

@@            Coverage Diff            @@
##           master    #233      +/-   ##
=========================================
- Coverage   88.21%   86.7%   -1.51%     
=========================================
  Files          15      15              
  Lines         509     504       -5     
=========================================
- Hits          449     437      -12     
- Misses         60      67       +7

Impacted Files	Coverage Δ
src/ModelingToolkit.jl	50% <ø> (-25%)	⬇️
src/utils.jl	88.15% <90.9%> (-3.85%)	⬇️
src/operations.jl	50% <0%> (-11.12%)	⬇️
src/variables.jl	89.47% <0%> (-5.36%)	⬇️
src/differentials.jl	93.02% <0%> (-0.16%)	⬇️
src/systems/diffeqs/diffeqsystem.jl	88.69% <0%> (+0.56%)	⬆️
src/direct.jl	100% <0%> (+6.25%)	⬆️

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[ChrisRackauckas]

Implements #133