Faster indexing for Ultraspherical(0.5) ⟷ Legendre

[jishnub]

After this,

julia> C = Conversion(Ultraspherical(0.5), Legendre());

julia> @btime $C[1:100, 1:100];
  169.329 ns (2 allocations: 944 bytes) # PR
  821.360 ns (6 allocations: 4.42 KiB) # master

This also fixes

julia> Conversion(NormalizedUltraspherical(1.5), NormalizedJacobi(1,1))
ConversionWrapper : NormalizedUltraspherical(1.5) → NormalizedJacobi(1,1)
 1.0000000000000002   ⋅                  …   ⋅                   ⋅   ⋅
  ⋅                  1.0000000000000002      ⋅                   ⋅   ⋅
  ⋅                   ⋅                      ⋅                   ⋅   ⋅
  ⋅                   ⋅                      ⋅                   ⋅   ⋅
  ⋅                   ⋅                      ⋅                   ⋅   ⋅
  ⋅                   ⋅                  …   ⋅                   ⋅   ⋅
  ⋅                   ⋅                      ⋅                   ⋅   ⋅
  ⋅                   ⋅                      ⋅                   ⋅   ⋅
  ⋅                   ⋅                     1.0000000000000004   ⋅   ⋅
  ⋅                   ⋅                      ⋅                  1.0  ⋅
  ⋅                   ⋅                  …   ⋅                   ⋅   ⋱

[codecov]

Codecov Report

Patch coverage: 98.03% and project coverage change: +0.58 🎉

Comparison is base (271f812) 69.70% compared to head (c327d47) 70.28%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #259      +/-   ##
==========================================
+ Coverage   69.70%   70.28%   +0.58%     
==========================================
  Files          28       28              
  Lines        2667     2679      +12     
==========================================
+ Hits         1859     1883      +24     
+ Misses        808      796      -12     

Impacted Files	Coverage Δ
src/Spaces/Jacobi/JacobiOperators.jl	85.11% <96.96%> (+4.77%)	⬆️
src/ApproxFunOrthogonalPolynomials.jl	90.00% <100.00%> (ø)
src/Spaces/Jacobi/Jacobi.jl	68.14% <100.00%> (ø)
src/Spaces/PolynomialSpace.jl	93.16% <100.00%> (-0.05%)	⬇️
...c/Spaces/Ultraspherical/UltrasphericalOperators.jl	80.18% <100.00%> (ø)

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