fix: make compiler happier in is_squarefree (#1669)

Fix the following compiler bug when using Hecke:
```
julia> @code_warntype is_squarefree(x)
MethodInstance for AbstractAlgebra.is_squarefree(::ZZPolyRingElem)
  from is_squarefree(f::PolyRingElem{<:RingElement}) @ Nemo ~/Nemo/src/HeckeMiscPoly.jl:517
Arguments
  #self#::Core.Const(AbstractAlgebra.is_squarefree)
  f::ZZPolyRingElem
Locals
  #1096::Nemo.var"#1096#1097"
  fac::Fac{ZZPolyRingElem}
Body::Any
```

Project.toml

@@ -1,6 +1,6 @@
 name = "Nemo"
 uuid = "2edaba10-b0f1-5616-af89-8c11ac63239a"
-version = "0.41.4"
+version = "0.41.5"
 
 [deps]
 AbstractAlgebra = "c3fe647b-3220-5bb0-a1ea-a7954cac585d"

src/HeckeMiscInteger.jl

@@ -233,7 +233,7 @@ function is_squarefree(n::Union{Int,ZZRingElem})
     if e > 1
         return false
     end
-    return isone(maximum(values(factor(n).fac)))
+    return isone(maximum(values(factor(n).fac); init = 1))
 end
 
 

src/HeckeMiscPoly.jl

@@ -516,7 +516,7 @@ end
 
 function is_squarefree(f::PolyRingElem{<:RingElement})
   iszero(f) && return false
-  degree(f) == 0 && return is_squarefree(leading_coefficient(f))
+  degree(f) == 0 && return is_squarefree(leading_coefficient(f))::Bool
   fac = factor_squarefree(f)
   return all(e <= 1 for (_, e) in fac)
 end