Port CLJ-2228: Improve performance of constantly (#321)

- Unroll constantly to arity 2
- Add test for constantly with four args

Co-authored-by: Erik Assum <erik@assum.net>

Author: swannodette

src/main/cljs/cljs/core.cljs

@@ -4462,7 +4462,11 @@ reduces them without incurring seq initialization"
 
 (defn constantly
   "Returns a function that takes any number of arguments and returns x."
-  [x] (fn [& args] x))
+  [x] (fn
+        ([] x)
+        ([_] x)
+        ([_ _] x)
+        ([_ _ & args] x)))
 
 (defn comp
   "Takes a set of functions and returns a fn that is the composition

src/test/cljs/cljs/other_functions_test.cljs

@@ -96,9 +96,14 @@
 (deftest test-constantly
   (let [c0 (constantly 10)]
     (are [x] (= 10 (c0 x))
-             nil
-             42
-             "foo")))
+         nil
+         42
+         "foo")
+    (are [x] (= 10 (c0 x :a :b :c))
+         nil
+         42
+         "foo")))
+
 ;juxt
 
 (deftest test-juxt