This commit introduces changes related to vault registrations in the
stake manager.
The stake manager needs to keep track of the vaults a users creates so
it can aggregate accumulated MP across vaults for any given user.
The `StakeVault` now comes with a `register()` function which needs to
be called to register itself with the stake manager. `StakeManager` has
a new `onlyRegisteredVault` modifier that ensures only registered vaults
can actually `stake` and `unstake`.
Closes#70
If there's a malicious upgrade which causes a stack overflow error when
`leave()` is called, the user of the vault should still be able to get
their funds out.
This commit adds a test that proofs this is happening.
