CrabBacking Module in Crab Runtime and CrabIssuing Module in Mainet Runtime

[hackfisher]

CRING are native tokens of Darwinia Crab Network, they will be able to transfer cross-chain from Crab to Mainet.

CrabBacking is a module on Crab Network for locking/redeeming the backing CRING, and there should be a counter-party CrabIssuing Module on Mainet for Issuing and Burning.

In mainet genesis, there are 40M RING stored in CrabIssuing module's account for supporting 1:100 converting between RING and CRING, the CRING will be locked in the CrabIssuing module's account after converted.

RING can be converted to CRING in CrabIssuing if and only if there are available CRING locked in the module account.

[hackfisher]

After rethinking the design, I would like to try anther way instead:

Using a Swap module and a Issuing(Mapped RING) module on Crab and a Backing(RING) module on Darwinia.

Closing this and open in another issue.

[hackfisher]

New issue moved to #509