[Feature Request][Move] Add set (collection) implementation

[alnoki]

@BriungRi @davidiw @movekevin @vgao1996 @wrwg

Presently, the Aptos Framework and Move/Aptos Standard libraries include assorted vector and map collections, but no set implementations. The following implementations are suggested:

For collection size fits in a single storage slot:

• A set based on an AVL tree, where keys are each of the int types, and nodes are vector entries
• A set based on an AVL tree, where keys are the hash of a generic type, and nodes are vector entries

For collection size does not fit in a single storage slot:

• "Smart" implementations, which automatically select the proper bucket size

AVL trees are suggested to optimize for lookup speed, though a red-black tree could be used for optimize insertion/removal time

[wrwg]

For this, we need ordering of arbitrary values T. We have discussed to implement this on native level (a hack), but we are also looking at actually adding traits (compiler v2). Probably we are moving forward with the hack (@lightmark FYI).