Rewrite filter.

lib/index.js

@@ -140,21 +140,10 @@ const Channel = function(length = 0) {
         }
       },
 
-      filter: (callbackfn, thisArg) => {
-        const output = Channel();
-
-        (async () => {
-          await readOnly.forEach(async (value) => {
-            if (await callbackfn.call(thisArg, value)) {
-              await output.push(value);
-            }
-          });
-
-          await output.close();
-        })();
-
-        return output;
-      },
+      filter: (callbackfn, thisArg) =>
+        readOnly.flatMap((value) =>
+          callbackfn.call(thisArg, value) ? Channel.of(value) : Channel.of()
+        ),
 
       flat: (depth) =>
         readOnly.flatMap((value) =>