Type-stability in 2d tensorizer iteration

[jishnub]

On master

julia> t = ApproxFunBase.tensorizer(Chebyshev()^2);

julia> @btime first(ApproxFunBase.cache($t), 100);
  26.686 μs (702 allocations: 58.25 KiB)

This PR

julia> @btime first(ApproxFunBase.cache($t), 100);
  16.425 μs (502 allocations: 47.31 KiB)

[codecov]

Codecov Report

Base: 71.39% // Head: 72.14% // Increases project coverage by +0.75% 🎉

Coverage data is based on head (acf6120) compared to base (fea3939).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #374      +/-   ##
==========================================
+ Coverage   71.39%   72.14%   +0.75%     
==========================================
  Files          79       79              
  Lines        8323     8314       -9     
==========================================
+ Hits         5942     5998      +56     
+ Misses       2381     2316      -65     

Impacted Files	Coverage Δ
src/LinearAlgebra/helper.jl	66.04% <100.00%> (+1.34%)	⬆️
src/Multivariate/TensorSpace.jl	87.91% <100.00%> (+0.93%)	⬆️
src/Operators/general/InterlaceOperator.jl	86.36% <0.00%> (+0.34%)	⬆️
src/Caching/almostbanded.jl	97.89% <0.00%> (+0.35%)	⬆️
src/PDE/KroneckerOperator.jl	79.60% <0.00%> (+0.39%)	⬆️
src/Space.jl	86.08% <0.00%> (+0.43%)	⬆️
src/LinearAlgebra/clenshaw.jl	39.47% <0.00%> (+0.52%)	⬆️
src/show.jl	98.47% <0.00%> (+0.76%)	⬆️
src/Multivariate/LowRankFun.jl	68.70% <0.00%> (+0.76%)	⬆️
... and 20 more

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