333 extend the orml tokens pallet so users are able to mint and burn custom assets

[gianfra-t]

Closes #333.

Note: This is still a very basic implementation. More functionality needs to be defined first.

General Overview

This pallet allows any user to "create" or "take" a CurrencyId:Token(u64), and define the controlling accounts.

Once taken, the owner can execute extrinsics mint and burn which calls orml_currencies functions deposit and withdraw respectively (this in turn will interact with orml_tokens as defined in the issue)

The StorageMap will hold the information about which CurrencyId:Token(u64) has already been taken, and the accounts that are able to mint, burn and transfer ownership.

Root Capabilities

Root can only transfer the ownership of the currency, which essentially allows to perform any operation.

[gianfra-t]

We should define some more things before moving forward. Some of them I can think of right now are:

• Do we need just the basic mint and burn functionality or we also need freezing assets, force transfer, etc.
• Do we want to allow for the destruction of a currency as it is done in assets_pallet? If so, how do we deal with token holders?

[ebma]

Looks good overall. We should probably add benchmarks but in my opinion we don't need to support freezing or anything fancy just yet.

[ebma]

Is this still a draft? Or is it ready for review now @gianfra-t?

[gianfra-t]

I believe it's ready. It does not have much functionality though but we decided to make destruction a different ticket as I recall.

[ebma]

Let's also require the owner of a currency to reserve a deposit when owning it, see here. This deposit cannot be unreserved until we add the 'destroy' feature in the future but it is very important because otherwise users could just claim ownership of all assets, squatting on them and distracting the usefulness of the feature overall. Note that the deposit reserve needs to be 'transferred' to the new owner when transferring ownership, see here, ie the new owner should then have to reserve some balance and it should be freed from the original owner.

[ebma]

@gianfra-t I added changes that remove the extra config type, please have a look. I did not re-generate the default weights yet, we should probably do that again now that we require the user to deposit some amount when claiming ownership.

[ebma]

Great 👍 ready for merge IMO