Support to Collections ("unique digital collection" or UDC)

Description

This PR implement the support UDC implementation on top of RGB21.

For now, I've only implemented the issuance, so I'm leaving it in draft. My goal is to complete the transfers in the next few days.

For help in test the issue subroutine, I documented all lines of the script and tested it with list bellow:

Validations (Checklist) Basically, we validate the contract item data and allocations:

[x] Amount of Items in tokens field (i.e. TokenData) must be equal than amount of items in assignments field (i.e. Allocation);
[x] Each index token registered in tokens field must have an allocation defined in assignments field;
[x] Each index token registered in assignments field must have an allocation defined in tokens field;
[x] Every allocation must have if and only if a fraction of the contract;
[x] If a token index is not found, the script must be failed.
[x] If a fraction is greater than 1, the script must be failed.

BTW, this PR depends other PRs:

RGB-WG / rgb-schemata