Avoid observe overhead for Future#map

Out of the combinators, this is the only case that does not rely on
pre-existing futures, auto detection, or rescuing errors, so the extra
verbosity seems like a reasonable trade-off.

lib/ione/future.rb

@@ -345,7 +345,13 @@ def map(value=nil, &block)
         else
           f = CompletableFuture.new
           on_complete do |v, e|
-            f.observe(map(value, &block))
+            if e
+              f.fail(e)
+            elsif block.nil?
+              f.resolve(value)
+            else
+              f.try(v, &block)
+            end
           end
           f
         end