Fix reductions so they don't overflow

Author: timholy

src/FixedPointNumbers.jl

@@ -60,6 +60,11 @@ include("ufixed.jl")
 include("deprecations.jl")
 
 
+# Promotions for reductions
+for F in (Base.AddFun, Base.MulFun)
+    @eval Base.r_promote{T}(::$F, x::FixedPoint{T}) = Float64(x)
+end
+
 # TODO: rewrite this by @generated
 for T in tuple(Fixed16, UF...)
     R = rawtype(T)

test/ufixed.jl

@@ -153,3 +153,6 @@ b, ad = scaledual(0.5, ad)
 generic_scale!(rfloat, a, 0.5)
 generic_scale!(rfixed, ad, b)
 @test rfloat == rfixed
+
+a = UFixed8[0xffuf8, 0xffuf8]
+@test sum(a) == 2.0