Slightly reduce allocations in Chebyshev Fun integrate

[jishnub]

On master

julia> f = Fun();

julia> @btime integrate($f)
  108.817 ns (3 allocations: 240 bytes)
Fun(Chebyshev(), [0.0, 0.0, 0.25])

This PR

julia> @btime integrate($f)
  73.764 ns (2 allocations: 160 bytes)
Fun(Chebyshev(), [0.0, 0.0, 0.25])

[codecov]

Codecov Report

Patch coverage: 91.42% and project coverage change: +22.70 🎉

Comparison is base (75383fe) 49.48% compared to head (c50f0e8) 72.18%.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #279       +/-   ##
===========================================
+ Coverage   49.48%   72.18%   +22.70%     
===========================================
  Files          28       29        +1     
  Lines        2696     2736       +40     
===========================================
+ Hits         1334     1975      +641     
+ Misses       1362      761      -601     

Impacted Files	Coverage Δ
src/ApproxFunOrthogonalPolynomials.jl	87.80% <ø> (+28.62%)	⬆️
src/ultraspherical.jl	61.84% <73.33%> (+11.84%)	⬆️
...c/Spaces/Ultraspherical/UltrasphericalOperators.jl	83.33% <87.50%> (+43.24%)	⬆️
src/Spaces/Chebyshev/Chebyshev.jl	62.89% <91.66%> (+12.18%)	⬆️
ext/ApproxFunOrthogonalPolynomialsStaticExt.jl	95.55% <95.55%> (ø)
src/Spaces/PolynomialSpace.jl	93.41% <100.00%> (+25.34%)	⬆️
src/Spaces/Spaces.jl	100.00% <100.00%> (+31.50%)	⬆️
src/Spaces/Ultraspherical/Ultraspherical.jl	86.00% <100.00%> (+60.13%)	⬆️
src/fastops.jl	100.00% <100.00%> (ø)
src/show.jl	57.89% <100.00%> (+7.89%)	⬆️

... and 18 files with indirect coverage changes

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