Precompute bandwidth in sumspace deriv

[jishnub]

After this, the following becomes type-stable:

julia> @inferred Derivative(Chebyshev()+Legendre())
DerivativeWrapper : Chebyshev() ⊕ Legendre() → Ultraspherical(1) ⊕ Jacobi(1,1)
 0.0  0.0  1.0   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
  ⋅   0.0  0.0  1.0   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
  ⋅    ⋅   0.0  0.0  2.0   ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
  ⋅    ⋅    ⋅   0.0  0.0  1.5   ⋅    ⋅    ⋅    ⋅   ⋅
  ⋅    ⋅    ⋅    ⋅   0.0  0.0  3.0   ⋅    ⋅    ⋅   ⋅
  ⋅    ⋅    ⋅    ⋅    ⋅   0.0  0.0  2.0   ⋅    ⋅   ⋅
  ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   0.0  0.0  4.0   ⋅   ⋅
  ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   0.0  0.0  2.5  ⋅
  ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   0.0  0.0  ⋱
  ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   0.0  ⋱
  ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋱

julia> @inferred Derivative(Chebyshev(0..1) + Legendre(1..2))
DerivativeWrapper : Chebyshev(0..1) ⨄ Legendre(1..2) → Ultraspherical(1,0..1) ⊕ Jacobi(1,1,1..2)
 0.0  0.0  2.0   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
  ⋅   0.0  0.0  2.0   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
  ⋅    ⋅   0.0  0.0  4.0   ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
  ⋅    ⋅    ⋅   0.0  0.0  3.0   ⋅    ⋅    ⋅    ⋅   ⋅
  ⋅    ⋅    ⋅    ⋅   0.0  0.0  6.0   ⋅    ⋅    ⋅   ⋅
  ⋅    ⋅    ⋅    ⋅    ⋅   0.0  0.0  4.0   ⋅    ⋅   ⋅
  ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   0.0  0.0  8.0   ⋅   ⋅
  ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   0.0  0.0  5.0  ⋅
  ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   0.0  0.0  ⋱
  ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   0.0  ⋱
  ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋱

[codecov]

Codecov Report

Patch coverage: 95.12% and project coverage change: +3.02 🎉

Comparison is base (8dbc982) 68.19% compared to head (7a3949c) 71.21%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #401      +/-   ##
==========================================
+ Coverage   68.19%   71.21%   +3.02%     
==========================================
  Files          80       80              
  Lines        8424     8471      +47     
==========================================
+ Hits         5745     6033     +288     
+ Misses       2679     2438     -241     

Impacted Files	Coverage Δ
src/ApproxFunBase.jl	83.33% <ø> (+12.50%)	⬆️
src/Operators/Operator.jl	67.02% <83.33%> (+1.76%)	⬆️
src/Operators/general/InterlaceOperator.jl	87.06% <100.00%> (+2.17%)	⬆️
src/Spaces/ProductSpaceOperators.jl	69.88% <100.00%> (-3.22%)	⬇️
src/show.jl	97.70% <100.00%> (+6.10%)	⬆️

... and 40 files with indirect coverage changes

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