Adding a simple coll? function to the core

* hy/core/language.hy:
  -Added a simple coll? function that checks whether the given argument
  is an iterable and not a string,
  - Also replaced the check in `flatten` by coll?

* tests/native_tests/core.hy: Tests updated for checking coll?

hy/core/language.hy

@@ -30,6 +30,10 @@
   (if (not (numeric? x))
     (raise (TypeError (.format "{0!r} is not a number" x)))))
 
+(defn coll? [coll]
+  "Checks whether item is a collection"
+  (and (iterable? coll) (not (string? coll))))
+
 (defn cycle [coll]
   "Yield an infinite repetition of the items in coll"
   (setv seen [])
@@ -112,7 +116,7 @@
 
 (defn flatten [coll]
   "Return a single flat list expanding all members of coll"
-  (if (and (iterable? coll) (not (string? coll)))
+  (if (coll? coll)
     (_flatten coll [])
     (raise (TypeError (.format "{0!r} is not a collection" coll)))))
 
@@ -298,8 +302,8 @@
   (_numeric_check n)
   (= n 0))
 
-(def *exports* '[calling-module-name cycle dec distinct disassemble drop
+(def *exports* '[calling-module-name coll? cycle dec distinct disassemble
-                 drop-while empty? even? filter flatten float? gensym
+		 drop drop-while empty? even? filter flatten float? gensym
                  inc instance? integer integer? iterable? iterate
                  iterator? macroexpand macroexpand-1 neg? nil? none?
                  nth numeric? odd? pos? remove repeat repeatedly second

tests/native_tests/core.hy

@@ -30,6 +30,14 @@
 (defn assert-equal [x y]
   (assert (= x y)))
 
+(defn test-coll? []
+  "NATIVE: testing coll?"
+  (assert-true (coll? [1 2 3]))
+  (assert-true (coll? {"a" 1 "b" 2}))
+  (assert-true (coll? (range 10)))
+  (assert-false (coll? "abc"))
+  (assert-false (coll? 1)))
+
 (defn test-cycle []
   "NATIVE: testing cycle"
   (assert-equal (list (cycle [])) [])