Move OP code from ApproxFun Here

[dlfivefifty]

I'm interested to see this package develop. It would be nice to move out as much of the ApproxFun OP code into this package. For example, we already have implemented the recurrence coefficients, Clenshaw 's algorithm, and various other OP functionality.

Here are the relevant files:

https://github.com/JuliaApproximation/ApproxFun.jl/blob/master/src/Spaces/PolynomialSpace.jl
https://github.com/JuliaApproximation/ApproxFun.jl/tree/master/src/Spaces/Hermite
https://github.com/JuliaApproximation/ApproxFun.jl/tree/master/src/Spaces/Jacobi
https://github.com/JuliaApproximation/ApproxFun.jl/blob/master/src/Spaces/Chebyshev/Chebyshev.jl
https://github.com/JuliaApproximation/ApproxFun.jl/tree/master/src/Spaces/Laguerre
https://github.com/JuliaApproximation/ApproxFun.jl/blob/master/src/LinearAlgebra/clenshaw.jl

I'm happy to help anyway I can.