This renames `RewardsStreamerMP` to `StakeManager`. The original name
only exists because the project has started with different versions of
the contract. Since the contract is no longer just dealing with MPs but
actually distributes rewards, it makes sense to make this the official
stake manager of the protocol.
**BREAKING CHANGE:**
- `RewardsStreamerMP` is now `StakeManager`
- `StakingManager__*` error selectors are now `StakeManager__*`
selectors
This adds a new emergency mode that can be enabled by the owner of the system.
When in emergency mode, stakers or `StakeVault`s can leave the system immediately.
This also applies when there was a malicious upgrade and a call to
`emergencyModeEnabled()` panics.
To have this in a fully secure manner, we still have to add the counter
part of "leaving" the system. This will allow users that don't agree
with a (malicious) upgrade to get their funds out of the vaults
regardless.
Closes#66
