Write a registrar module

[rphmeier]

This supersedes the functionality previously implemented in the registrar module of the v0 runtime. The ways in which it is superseded are described in #1501 .

This is point 1: This Registrar should provide a small wrapper around the Paras for handling deposits.

Things this module needs to do:

• Accept parathread registrations and forward to the Paras module. Hold deposits for parathread
• Accept parachain registrations and forward to the Paras module
• Handle de-registrations of parathreads and parachains
• Expose a Call for Parachain origins to trigger parachain/thread swaps (Add an origin for Parachains #1510, Parachain/thread swapping #1509)

The old registrar code (prior to #1501) may provide a reference for some of the logic