Add `extend string` to generic types used in object key position

[gobanos]

The following Rust struct

#[derive(Serialize, Deserialize)]
pub struct Foo<Key, Value>
where
    Key: Eq + Ord,
{
    pub foos: BTreeMap<Key, Value>,
}

is translated to this invalid typescript :

export interface Foo<Key, Value> {
    foos: Record<Key, Value> // <- Type 'Key' does not satisfy the constraint 'string | number | symbol'.
}

The expected typescript is :

export interface Foo<Key extends string, Value> {
    foos: Record<Key, Value>
}

[AlisCode]

So :

• We need to change the Map exporter
• We need to support extends OtherType in ts_json_subset
• We need to provide a way to apply a constraint on generics (such as extends OtherType). This means that :
  • Generics must be passed mutably to the solvers. Beware : potential updates on generics must not be applied if the solver fails (or returns Continue), otherwise solvers might interfere with each other and we don't want that
  • Generics can no longer be syn's Generics type, as we need to store the constraint in there somehow