New stack design

[GiggleLiu]

With improved performance for scalar, built into the compiler.

[codecov]

Codecov Report

Merging #59 (83781c4) into master (aba8b80) will decrease coverage by 0.54%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
- Coverage   79.75%   79.21%   -0.55%     
==========================================
  Files          28       28              
  Lines        1245     1227      -18     
==========================================
- Hits          993      972      -21     
- Misses        252      255       +3     

Impacted Files	Coverage Δ
src/NiLang.jl	50.00% <ø> (ø)
src/autodiff/autodiff.jl	100.00% <ø> (ø)
src/stdlib/blas.jl	48.88% <37.50%> (-1.12%)	⬇️
src/stdlib/sparse.jl	79.59% <65.00%> (-13.59%)	⬇️
src/autodiff/stack.jl	71.42% <71.42%> (ø)
src/stdlib/nnlib.jl	86.11% <77.77%> (-5.07%)	⬇️
src/instructs.jl	91.02% <93.75%> (+0.70%)	⬆️
src/autodiff/vars.jl	77.57% <100.00%> (+3.65%)	⬆️
src/stdlib/mapreduce.jl	100.00% <100.00%> (ø)
src/stdlib/sorting.jl	100.00% <100.00%> (ø)
... and 3 more

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