aggressive constprop in intpow (#206)

Author: jishnub

src/Fun.jl

@@ -353,8 +353,12 @@ end
 
 \(c::Number, f::Fun) = Fun(f.space, c \ f.coefficients)
 
-
-function intpow(f::Fun,k::Integer)
+@static if VERSION >= v"1.8"
+    Base.@constprop :aggressive intpow(f::Fun, k::Integer) = _intpow(f, k)
+else
+    intpow(f::Fun, k::Integer) = _intpow(f, k)
+end
+@inline function _intpow(f, k)
     if k == 0
         ones(cfstype(f), space(f))
     elseif k==1