[NPoS] Unify staking functions across staking pallet and nomination pallet

[Ank4n]

Nomination pool is a way to scale nominators and is only an implementation detail for a user who wants to stake their dots. Once we have a proper in-place fund holding mechanism with #454, we should look into unifying these two separate pallet functions in a common entry point.

This could be done at Nomination pool pallet since its already an abstraction over staking pallet.

Most of staking pallet functions could be disabled through extrinsics. Instead Nomination pool could expose the following calls:

1. Stake via a existing pool
2. Stake directly
   2A) Allow others to pool in your nominations.
   2B) Do not allow others.

Functionally, a direct nomination could be seen as just a pool where only the pool owner stakes and no one is allowed to join a pool (2B). 2A would be similar to creating a pool and being pool owner for that.

Unifying apis could help eventually to have xcm instructions for staking as well.