Specialize isdiag for certain ConversionWrappers

[jishnub]

After this,

julia> C = Conversion(Chebyshev(), Ultraspherical(2));

julia> using LinearAlgebra

julia> @code_typed isdiag(C) # compile-time constant
CodeInfo(
1 ─     return false
) => Bool

[codecov]

Codecov Report

Patch coverage: 85.71% and project coverage change: +0.09% 🎉

Comparison is base (c4d7be5) 69.01% compared to head (5ca8997) 69.11%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #287      +/-   ##
==========================================
+ Coverage   69.01%   69.11%   +0.09%     
==========================================
  Files          29       29              
  Lines        2737     2749      +12     
==========================================
+ Hits         1889     1900      +11     
- Misses        848      849       +1     

Files Changed	Coverage Δ
src/ApproxFunOrthogonalPolynomials.jl	82.50% <ø> (ø)
...c/Spaces/Ultraspherical/UltrasphericalOperators.jl	79.82% <77.77%> (+0.19%)	⬆️
src/Spaces/Jacobi/JacobiOperators.jl	83.54% <100.00%> (+0.26%)	⬆️

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