Split std files

compiler/erg_compiler/lib/std/_erg_str.py

@@ -0,0 +1,10 @@
+from _erg_result import Error
+
+class Str(str):
+    def __instancecheck__(cls, obj):
+        return isinstance(obj, str)
+    def try_new(s: str): # -> Result[Nat]
+        if isinstance(s, str):
+            return Str(s)
+        else:
+            return Error("Str can't be other than str")