Add a method for casting into byte string or unicode depending on python version

hy/core/language.hy

@@ -177,6 +177,12 @@
   "Return second item from `coll`"
   (get coll 1))
 
+(defn string [x]
+  "Cast x as current string implementation"
+  (if-python2
+   (unicode x)
+   (str x)))
+
 (defn string? [x]
   "Return True if x is a string"
   (if-python2
@@ -218,5 +224,5 @@
                 "even?" "filter" "float?" "inc"
                 "instance?" "integer?" "iterable?" "iterate" "iterator?" "neg?"
                 "none?" "nth" "numeric?" "odd?" "pos?" "remove" "repeat"
-                "repeatedly" "second" "string?" "take" "take_nth" "take_while"
+                "repeatedly" "second" "string" "string?" "take" "take_nth" "take_while"
                 "zero?"])

tests/native_tests/language.hy

@@ -779,3 +779,7 @@
   "Evaluate an empty list to a []"
   (assert (= () [])))
 
+(defn test-string []
+  (assert (string? (string "a")))
+  (assert (string? (string 1)))
+  (assert (= u"unicode" (string "unicode"))))