[manifest] Add a `Manager` type for managing manifest updates

[mcy]

This type is analogous to a similar class of manager types in the Microsoft C implementation, whose primary purpose is to perform the necessary checks for activating a manifest.

Right now, we expect the caller to track the "staging" area separately, though it may wind up making sense to move that into Manager in the fullness of time.

[mcy]

+jrvanwhy because there's some trait grossness here.