RGB: collectible schema proposal

[dr-orlovsky]

Closes #17

• Creator of the asset MAY limit total number of issues
• A single asset is issued per seal, always (indivisibility)
• Each transfer is a transfer of a single item-per-seal (direct consequence)
• No burn&reissue procedure
• Issue may contain binary metadata

Note:
Asset data are limited to 2^16 bytes; for larger asset a reference/link to the externally stored resource (like IPFS) should be used. This is related to general data size limitations of RGB system.

Opened questions:

• should we allow owners of the asset add some additonal binary data to the as the "asset history" (like adding engravings to the owned item)

Update:

• Unique tokens & token-specfic data
• Issue control
• Generic token data, internal or external, in different formats
• Engravings (any why Schema subclassing is required)
• LockUTXOs and descriptors for proof of reserves
• Renominations
• Rights splits