Fixed LHS auto typing conversion for Expr arithmetic (#146)

Now Expr arithmetic doesn't require LHS to be Expr but can be ToExpr as
well.

Thanks to Sandro for pointing out this error.

Author: qwang98

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "chiquito"
-version = "0.1.2023092400"
+version = "0.1.2023101100"
 edition = "2021"
 license = "MIT OR Apache-2.0"
 authors = ["Leo Lara <[email protected]>"]

src/frontend/python/chiquito/expr.py

@@ -26,20 +26,23 @@ def __add__(self: Expr, rhs: ToExpr) -> Sum:
         return Sum([self, rhs])
 
     def __radd__(self: Expr, lhs: ToExpr) -> Sum:
+        lhs = to_expr(lhs)
         return Expr.__add__(lhs, self)
 
     def __sub__(self: Expr, rhs: ToExpr) -> Sum:
         rhs = to_expr(rhs)
         return Sum([self, Neg(rhs)])
 
     def __rsub__(self: Expr, lhs: ToExpr) -> Sum:
+        lhs = to_expr(lhs)
         return Expr.__sub__(lhs, self)
 
     def __mul__(self: Expr, rhs: ToExpr) -> Mul:
         rhs = to_expr(rhs)
         return Mul([self, rhs])
 
     def __rmul__(self: Expr, lhs: ToExpr) -> Mul:
+        lhs = to_expr(lhs)
         return Expr.__mul__(lhs, self)
 
     def __pow__(self: Expr, rhs: int) -> Pow: