Improve type-inference in TimesOperator * Fun

[jishnub]

After this,

julia> S = NormalizedUltraspherical(NormalizedLegendre())
NormalizedUltraspherical(1/2)

julia> r = Fun(S)
Fun(NormalizedUltraspherical(1/2), [0.0, 0.816497])

julia> @inferred (r -> r^2)(r)
Fun(NormalizedUltraspherical(1/2), [0.471405, 0.0, 0.421637])

[codecov]

Codecov Report

Patch coverage: 89.83% and project coverage change: -0.75 ⚠️

Comparison is base (2f401fa) 71.22% compared to head (9262f13) 70.48%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #481      +/-   ##
==========================================
- Coverage   71.22%   70.48%   -0.75%     
==========================================
  Files          80       81       +1     
  Lines        8331     8470     +139     
==========================================
+ Hits         5934     5970      +36     
- Misses       2397     2500     +103     

Impacted Files	Coverage Δ
src/ApproxFunBase.jl	84.00% <ø> (+0.66%)	⬆️
src/LinearAlgebra/blas.jl	77.77% <ø> (ø)
src/Spaces/Spaces.jl	63.52% <ø> (-3.53%)	⬇️
src/Operators/qr.jl	57.89% <22.22%> (-2.82%)	⬇️
src/Spaces/SumSpace.jl	70.44% <33.33%> (+0.77%)	⬆️
src/Operators/SubOperator.jl	82.35% <50.00%> (-0.82%)	⬇️
src/Operators/banded/CalculusOperator.jl	75.40% <80.64%> (+0.84%)	⬆️
src/Operators/functionals/CalculusFunctional.jl	79.48% <86.66%> (+7.07%)	⬆️
src/eigen.jl	96.42% <96.42%> (ø)
src/show.jl	95.68% <97.56%> (+12.85%)	⬆️
... and 6 more

... and 44 files with indirect coverage changes

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