Add Collection module to fastx framework

[lxfind]

This PR adds Collection, a heterogeneous object collection that owns a list of objects with potentially different types. This is a modules that demonstrates how object ownership can be used.

There are some other issues preventing us from exposing the add function as entry points: #348

[lxfind]

Made a few changes:

1. Added remove_and_take to Collection. Now remove will return the object being removed, and it's no longer an entry function; while remove_and_take can be an entry function and always transfer the object to the signer.
2. Call Collection::transfer instead of Transfer::transfer in the test.

[gdanezis]

I have a question here: so we allow an owner to add an object to a collection, which is very cool. However I think that it is possible for the owner of the collection object to transfer the object owned to someone else?

    // Remove the object from the collection. After the removal,
    /// the object is still owned by the collection. Caller will need
    /// to decide where to transfer it to.

What prevents the owner of the collection from moving the object in a collection, while it is still in the collection? Would it not be safer to wrap the object in the collection into a special object that denotes items in this collection, and unwrap it when it comes out of the collection, and forbid it moving while it is still in the collection?

[sblackshear]

I have a question here: so we allow an owner to add an object to a collection, which is very cool. However I think that it is possible for the owner of the collection object to transfer the object owned to someone else?

    // Remove the object from the collection. After the removal,
    /// the object is still owned by the collection. Caller will need
    /// to decide where to transfer it to.

What prevents the owner of the collection from moving the object in a collection, while it is still in the collection? Would it not be safer to wrap the object in the collection into a special object that denotes items in this collection, and unwrap it when it comes out of the collection, and forbid it moving while it is still in the collection?

Every function that can transfer an object requires Move ownership (e.g., a T instead of &mut T) and makes the transferred object unreachable for the rest of the transaction. Because you can't get Move ownership of an item in the collection without also removing it, it either has to be in one place or the other.

I do think the comment is potentially confusing, though... Every object remains owned by whoever owned it at the beginning of the transaction unless it is explicitly passed to a transfer (in which case it will be unreachable for the rest of the transaction, so no ownership change can be observed by the program). Said more concisely, object ownership is invariant within a single transaction, but the comment makes it sound like this might not be true

[lxfind]

Addressed feedback:

1. Added contains method
2. Added checks to avoid double add