Impove type inference in RaggedMatrix constructor

[jishnub]

This avoids recursion in the RaggedMatrix constructor from SubOperators.
After this,

julia> d1, r1 = Legendre(), Jacobi(2,2);

julia> d2, r2 = Chebyshev(), Chebyshev();

julia> K = (Operator(I, d1) ⊗ Operator(I, d2)) → (r1 ⊗ r2)
KroneckerOperator : Legendre() ⊗ Chebyshev() → Jacobi(2,2) ⊗ Chebyshev()
 1.0  0.0  0.0       0.0  0.0       -0.285714  0.0  0.0        0.0        0.0       ⋯
  ⋅   1.0  0.0       0.0  0.0        0.0       0.0  0.0       -0.285714   0.0       ⋱
  ⋅   0.0  0.333333  0.0  0.0        0.0       0.0  0.0        0.0       -0.222222  ⋱
  ⋅    ⋅    ⋅        1.0  0.0        0.0       0.0  0.0        0.0        0.0       ⋱
  ⋅    ⋅    ⋅        0.0  0.333333   0.0       0.0  0.0        0.0        0.0       ⋱
  ⋅    ⋅    ⋅        0.0  0.0        0.214286  0.0  0.0        0.0        0.0       ⋱
  ⋅    ⋅    ⋅         ⋅    ⋅          ⋅        1.0  0.0        0.0        0.0       ⋱
  ⋅    ⋅    ⋅         ⋅    ⋅          ⋅        0.0  0.333333   0.0        0.0       ⋱
  ⋅    ⋅    ⋅         ⋅    ⋅          ⋅        0.0  0.0        0.214286   0.0       ⋱
  ⋅    ⋅    ⋅         ⋅    ⋅          ⋅        0.0  0.0        0.0        0.166667  ⋱
  ⋮    ⋱    ⋱         ⋱    ⋱          ⋱         ⋱    ⋱          ⋱          ⋱        ⋱

julia> @inferred K[1:1, 1:1]
1×1 ApproxFunBase.RaggedMatrix{Float64}:
 1.0

[codecov]

Codecov Report

Patch coverage: 68.33% and project coverage change: -44.48% ⚠️

Comparison is base (4f33475) 71.88% compared to head (5890748) 27.40%.
Report is 12 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #520       +/-   ##
===========================================
- Coverage   71.88%   27.40%   -44.48%     
===========================================
  Files          81       80        -1     
  Lines        8546     8360      -186     
===========================================
- Hits         6143     2291     -3852     
- Misses       2403     6069     +3666     

Files Changed	Coverage Δ
src/ApproxFunBase.jl	44.00% <ø> (-40.00%)	⬇️
src/Operators/SubOperator.jl	42.64% <0.00%> (-41.18%)	⬇️
src/Operators/banded/CalculusOperator.jl	17.35% <0.00%> (-58.46%)	⬇️
src/Operators/banded/Conversion.jl	22.95% <0.00%> (-63.49%)	⬇️
src/Spaces/ProductSpaceOperators.jl	0.00% <0.00%> (-65.37%)	⬇️
src/eigen.jl	0.00% <0.00%> (-96.43%)	⬇️
src/Fun.jl	36.17% <50.00%> (-31.62%)	⬇️
src/Operators/general/InterlaceOperator.jl	23.46% <66.66%> (-57.87%)	⬇️
src/LinearAlgebra/helper.jl	55.58% <100.00%> (-13.35%)	⬇️
src/Multivariate/TensorSpace.jl	39.83% <100.00%> (-45.34%)	⬇️
... and 2 more

... and 60 files with indirect coverage changes

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