fix Stacked by fusing two loops into one in a Zygote-friendly way

src/bijectors/stacked.jl

@@ -230,15 +230,11 @@ end
 end
 
 function _with_logabsdet_jacobian(sb::Stacked, x::AbstractVector)
-    ys = map(zip(sb.bs, sb.ranges_in)) do tup
-        b, r = tup[1], tup[2]
-        b(x[r])
-    end
-    y = reduce(vcat, ys)
-    logjac = sum(zip(sb.bs, sb.ranges_in)) do tup
-        b, r = tup[1], tup[2]
-        logabsdetjac(b, x[r])
+    ys_and_logjacs = map(zip(sb.bs, sb.ranges_in)) do (b, r)
+        with_logabsdet_jacobian(b, x[r])
     end
+    y = reduce(vcat, map(first, ys_and_logjacs))
+    logjac = sum(map(last, ys_and_logjacs))
     return (y, logjac)
 end