Make fun immutable (#136)

* make fun immutable

* test against AFSingularities

Author: jishnub

.github/workflows/downstream.yml

@@ -22,6 +22,7 @@ jobs:
           - {repo: ApproxFun.jl, group: JuliaApproximation}
           - {repo: ApproxFunOrthogonalPolynomials.jl, group: JuliaApproximation}
           - {repo: ApproxFunFourier.jl, group: JuliaApproximation}
+          - {repo: ApproxFunSingularities.jl, group: JuliaApproximation}
 
     steps:
       - uses: actions/checkout@v2

src/Fun.jl

@@ -8,7 +8,7 @@ include("Space.jl")
 ##  Constructors
 
 
-mutable struct Fun{S,T,VT} <: Function
+struct Fun{S,T,VT} <: Function
     space::S
     coefficients::VT
     function Fun{S,T,VT}(sp::S,coeff::VT) where {S,T,VT}