This commit introduces a first version of a `VaultFactory` that later
will be extended to be capable of instantiating reward vaults and
possible keep track of vault instances per owner.
As a first step, this implementation comes with a `createVault()`
function which takes care of creating vaults.
Because `VaultFactory` also knows about `StakeManager` it can derive the
manager's address and stake token from it when creating vaults, allowing
the API to be without arguments.
Partially addresses #37
Because the `stakedToken` property is `immutable`, solhint recommends to
make it in all caps. This commit changes the property to adhere to that
rule and also makes the property private.
To access the `stakedToken` there's now a `stakedToken()` function on
the contract.
r4bbit