Add @functorize(centralizedabs2fun)

Note that `@functorize(centralizedabs2fun)` returns the type which can
then be used to construct the actual functor as e.g.
`@functorize(centralizedabs2fun)(23)`.

src/Compat.jl

@@ -983,6 +983,7 @@ macro functorize(f)
     if VERSION >= v"0.5.0-" && !isdefined(Base, :BitFunctorUnary)
         f === :scalarmax       ? :(Base.scalarmax) :
         f === :scalarmin       ? :(Base.scalarmin) :
+        f === :centralizedabs2fun ? :(typeof(Base.centralizedabs2fun(0)).name.primary) :
         f
     else
         f = f === :identity        ? :(Base.IdFun()) :
@@ -996,6 +997,7 @@ macro functorize(f)
             f === :*               ? :(Base.MulFun()) :
             f === :scalarmax       ? :(Base.MaxFun()) :
             f === :scalarmin       ? :(Base.MinFun()) :
+            f === :centralizedabs2fun ? :(Base.CentralizedAbs2Fun) :
             f
         if VERSION >= v"0.4.0-dev+4902"
             f = f === :<           ? :(Base.LessFun()) :

test/runtests.jl

@@ -1060,3 +1060,8 @@ for (Fun, func) in [(:AndFun,            :&),
         end
     end
 end
+@test isa(@functorize(centralizedabs2fun)(1), @functorize(centralizedabs2fun))
+@test isa(@functorize(centralizedabs2fun)(1.0), @functorize(centralizedabs2fun))
+let a = rand(1:10, 10)
+    @eval @test mapreduce(x -> abs2(x - 1), +, $(a)) == mapreduce(@functorize(centralizedabs2fun)(1), +, $(a))
+end