Implement ADR 4 (part 1, runtime descriptors)

[abukosek]

Runtime descriptors, runtime parameters, and the genesis document structures should be changed as described in https://github.com/oasisprotocol/oasis-core/blob/master/docs/adr/0004-runtime-governance.md.
Additional changes to support the new structures are required in the registry, staking, and scheduler.

Old descriptors are accepted and converted into the new format at genesis time, but all new runtime registrations must use the new descriptor format.

After this issue, as well as issues #3429 and #3430 are complete, the rest of the ADR (messages) can be implemented. A separate issue will be made for that later.