Inference in Chebyshev/Ultraspherical conversion

[jishnub]

This improves the type inference, as the result becomes a union of concrete types.
Master:

julia> @code_warntype Conversion(Chebyshev(), Ultraspherical(1))
MethodInstance for Conversion(::Chebyshev{ChebyshevInterval{Float64}, Float64}, ::Ultraspherical{Int64, ChebyshevInterval{Float64}, Float64})
  from Conversion(A::Chebyshev, B::Ultraspherical) in ApproxFunOrthogonalPolynomials at /home/jishnu/Dropbox/JuliaPackages/ApproxFunOrthogonalPolynomials.jl/src/Spaces/Ultraspherical/UltrasphericalOperators.jl:120
Arguments
  #self#::Type{Conversion}
  A::Core.Const(Chebyshev())
  B::Ultraspherical{Int64, ChebyshevInterval{Float64}, Float64}
Locals
  US::Ultraspherical{Int64, ChebyshevInterval{Float64}, Float64}
  d::ChebyshevInterval{Float64}
Body::Union{ApproxFunBase.ConcreteConversion{Chebyshev{ChebyshevInterval{Float64}, Float64}, Ultraspherical{Int64, ChebyshevInterval{Float64}, Float64}, Float64}, ApproxFunBase.ConversionWrapper}

This PR:

julia> @code_warntype Conversion(Chebyshev(), Ultraspherical(1))
MethodInstance for Conversion(::Chebyshev{ChebyshevInterval{Float64}, Float64}, ::Ultraspherical{Int64, ChebyshevInterval{Float64}, Float64})
  from Conversion(A::Chebyshev, B::Ultraspherical) in ApproxFunOrthogonalPolynomials at /home/jishnu/Dropbox/JuliaPackages/ApproxFunOrthogonalPolynomials.jl/src/Spaces/Ultraspherical/UltrasphericalOperators.jl:120
Arguments
  #self#::Type{Conversion}
  A::Core.Const(Chebyshev())
  B::Ultraspherical{Int64, ChebyshevInterval{Float64}, Float64}
Locals
  US::Ultraspherical{Int64, ChebyshevInterval{Float64}, Float64}
  d::ChebyshevInterval{Float64}
Body::Union{ApproxFunBase.ConcreteConversion{Chebyshev{ChebyshevInterval{Float64}, Float64}, Ultraspherical{Int64, ChebyshevInterval{Float64}, Float64}, Float64}, ApproxFunBase.ConversionWrapper{TimesOperator{Float64, Tuple{Int64, Int64}}, Float64}}

Tests are not ideal here, as they will pass on master as well, but at least they'll prevent against egregious regressions.

[codecov]

Codecov Report

Merging #88 (10de42a) into master (fa98e6b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #88   +/-   ##
=======================================
  Coverage   57.60%   57.60%           
=======================================
  Files          27       27           
  Lines        2413     2413           
=======================================
  Hits         1390     1390           
  Misses       1023     1023           

Impacted Files	Coverage Δ
...c/Spaces/Ultraspherical/UltrasphericalOperators.jl	75.92% <100.00%> (ø)

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