hugr-model represents constants for a runtime type ?t as terms of the static type (core.const ?t). To ease the migration from JSON encoded to term encoded constants, we should add the ability to represent the type of constants in TypeParam.

This could be done, as it is in hugr-model, via custom constructors. However, to avoid having to figure out custom TypeParams first (#2296), we can special case this (for now) as follows:

pub enum TypeParam {
    // ...

    /// The type of runtime constants.
    Const {
        runtime_type: Type,
    }
}

In particular, resolving this issue would require:

• Add a Const variant to TypeParam as above.
• Add a ConstTypeParam to the Python bindings.
• Write the Rust model to core import code that imports (core.const ?t) as TypeParam::Const.
• Write the Rustcore to model export code that exports TypeParam::Const to (core.const ?t).
• Write the Python core to model export code that exports ConstTypeParam to (core.const ?t).
• Update the spec accordingly.