Make runtime string concatenation and repetition work

Author: namannimmo10

integration_tests/test_builtin_str_02.py

@@ -1,3 +1,5 @@
+from ltypes import i32
+
 def _lpython_strcmp_eq(a: str, b: str) -> bool:
     if len(a) != len(b):
         return False
@@ -38,6 +40,17 @@ def _lpython_strcmp_gt(a: str, b: str) -> bool:
 def _lpython_strcmp_gteq(a: str, b: str) -> bool:
     return _lpython_strcmp_eq(a, b) or _lpython_strcmp_gt(a, b)
 
+def _lpython_str_concat(a: str, b: str) -> str:
+    return a + b
+
+def _lpython_str_repeat(a: str, n: i32) -> str:
+    s: str
+    s = ""
+    i: i32
+    for i in range(n):
+        s += a
+    return s
+
 def f():
     assert _lpython_strcmp_eq("a", "a")
     assert not _lpython_strcmp_eq("a2", "a")
@@ -80,4 +93,9 @@ def f():
     assert _lpython_strcmp_gteq("bg", "abc")
     assert _lpython_strcmp_gteq(a, b)
 
+    assert _lpython_str_concat("abc", "def") == "abcdef"
+    assert _lpython_str_concat("", "z") == "z"
+    assert _lpython_str_repeat("abc", 3) == "abcabcabc"
+    assert _lpython_str_repeat("", -1) == ""
+
 f()