Fix missing prefix bound types

[maurolacy]

Fixes a couple of errors with missing bound annotations for IndexedMaps and MultiIndex.

[ueco-jb]

Ugh, so what is 3rd type argument in Prefix again?

pub struct Prefix<K, T, B = Vec<u8>>
where
    K: KeyDeserialize,
    T: Serialize + DeserializeOwned,
{
    storage_prefix: Vec<u8>,
    data: PhantomData<(T, B)>,
    pk_name: Vec<u8>,
    de_fn_kv: DeserializeKvFn<K, T>,
    de_fn_v: DeserializeVFn<T>,
}

It's the actual content of prefix, right?

[maurolacy]

The 3rd type is precisely the new Bound type annotations. You can compare with Map to get the idea.

While implementing this, made it work for Map, and then forgot about these here and elsewhere (MultiIndex). These bounds have a default of Vec<u8>, that's why it "worked" (passed existing tests).

Adding explicit tests for these typed bounds would be good I guess, at some point.

[maurolacy]

It's a little tricky how all this works. I can give a small overview / talk at some point, if there's interest.

[ueco-jb]

Adding explicit tests for these typed bounds would be good I guess, at some point.

I think this is worth an extra issue as follow up.