Make Expr::eval_to_const public for FixedRule.

[thipokch]

Context

I'm writing a custom fixed rules with parameters/options. I am able to get scalar value (bool, int, string...) from options just fine.

Problem

With Lists, I can extract the values using expr_option(), which returns Expr. However, I am unable to use value from Expr, because it has yet to be evaluated Const variant using eval_to_const().

Solution

1. Make Expr::eval_to_const public
2. Evaluate expression to const for FixedRule.

[thipokch]

@zh217 Thank you :)