Fix builtin types methods

crates/erg_compiler/lib/std/_erg_convertors.py

@@ -1,5 +1,7 @@
 from _erg_int import Int
 from _erg_nat import Nat
+from _erg_float import Float
+from _erg_str import Str
 
 def int__(i):
     try:
@@ -12,3 +14,15 @@ def nat__(i):
         return Nat(i)
     except:
         return None
+
+def float__(f):
+    try:
+        return Float(f)
+    except:
+        return None
+
+def str__(s):
+    try:
+        return Str(s)
+    except:
+        return None