add typetraits.elementType (#14780)

* add typetraits.elementType

* add a test for 0-sized seq

lib/pure/typetraits.nim

@@ -93,6 +93,18 @@ since (1, 1):
   type StaticParam*[value: static type] = object
     ## used to wrap a static value in `genericParams`
 
+since (1, 3, 5):
+  template elementType*(a: untyped): typedesc =
+    ## return element type of `a`, which can be any iterable (over which you
+    ## can iterate)
+    runnableExamples:
+      iterator myiter(n: int): auto =
+        for i in 0..<n: yield i
+      doAssert elementType(@[1,2]) is int
+      doAssert elementType("asdf") is char
+      doAssert elementType(myiter(3)) is int
+    typeof(block: (for ai in a: ai))
+
 import std/macros
 
 macro genericParamsImpl(T: typedesc): untyped =