make String an implicit ns like Math. revert char? and clarify docstr…

…ing. add unit tests for char?

src/main/cljs/cljs/core.cljs

@@ -229,9 +229,9 @@
   (goog/isString x))
 
 (defn ^boolean char?
-  "Returns true if x is a JavaScript char."
+  "Returns true if x is a JavaScript string of length one."
   [x]
-  (gstring/isUnicodeChar x))
+  (and (string? x) (== 1 (.-length x))))
 
 (defn ^boolean any?
   "Returns true if given any argument."

src/main/clojure/cljs/analyzer.cljc

@@ -648,7 +648,7 @@
             (throw (error ~env (.getMessage err#) err#)))))))
 
 ;; namespaces implicit to the inclusion of cljs.core
-(def implicit-nses '#{goog goog.object goog.string goog.array Math})
+(def implicit-nses '#{goog goog.object goog.string goog.array Math String})
 
 (defn implicit-import?
   #?(:cljs {:tag boolean})

src/test/cljs/cljs/primitives_test.cljs

@@ -950,4 +950,14 @@
     (is (= 1 (do #js {:a 1}
                  1)))
     (is (= 1 (aget #js {:a 1} "a")))
-    (is (= 1 (.-a #js {:a 1})))))
+    (is (= 1 (.-a #js {:a 1})))))
+
+(deftest test-char?
+  (is (char? "0"))
+  (is (char? (String/fromCharCode 13)))
+  (is (char? (String/fromCharCode 10)))
+  (is (char? \newline))
+  (is (char? \space))
+  (is (char? "0"))
+  (is (char? "\u0080"))
+  (is (char? "\uFFFD")))