Add copy helper function to list (#4)

* Add copy helper function to list

* Add list.copy test

Co-authored-by: ‘markhend’ <‘markhend@gmail.com’>

stdlib/internal/types/collections/list.codon

@@ -328,6 +328,9 @@ class List:
             self[j] = x
             i += 1
 
+    def copy(self):
+        return self.__copy__()
+
 # Internal helpers
 
     def _idx_check(self, idx: int, msg: str):

test/core/containers.codon

@@ -115,6 +115,9 @@ def test_list():
     l6.extend(['xyz'])
     l6.extend('')
     assert l6 == ['a', 'b', 'c', 'xyz']
+
+    assert List[int]().copy() == List[int]()
+    assert [1,2,3].copy() == [1,2,3]
 test_list()
 
 @test