feat(subscription): support generics in MergedSubscription types

[mechiru]

Currently, defining a MergedSubscription that contains generics causes errors.

    #[derive(MergedSubscription)]
    struct Subscription<T1, T2>(Subscription1<T1>, Subscription2<T2>)
    where
        T1: Clone + OutputType,
        T2: Clone + OutputType;

   --> tests/merged_object.rs:219:12
    |
219 |     struct Subscription<T1, T2>(Subscription1<T1>, Subscription2<T2>)
    |            ^^^^^^^^^^^^ expected 2 generic arguments
    |
note: struct defined here, with 2 generic parameters: `T1`, `T2`
   --> tests/merged_object.rs:219:12
    |
219 |     struct Subscription<T1, T2>(Subscription1<T1>, Subscription2<T2>)
    |            ^^^^^^^^^^^^ --  --
help: add missing generic arguments
    |
219 |     struct Subscription<T1, T2><T1, T2>(Subscription1<T1>, Subscription2<T2>)
    |            ~~~~~~~~~~~~~~~~~~~~