allow use custom's const field to declare symbolic size

[katat]

This makes it able to declare methods to return array using the struct's field as its symbolic size.

For example:

fn Thing.to_array(self) -> [Field; self.cst] {
    return [1; self.cst];
}

Changes

1. Refactored the constant propagation to allow propagating multiple constants, such as array and custom type, in addition to single constant.
2. Allow propagating constant values from native function/method.

I also create an issue to propose supporting const attribute for custom fields for better type checking. So it can type check self.cst for [Field; self.cst] where the symbolic size has to be constant.

[mimoo]

as discussed privately I don't think we should go this route :o

[katat]

agreed, closing this.