[state] Consolidate state handling in Vernacstate

After #12504 , we can encapsulate and consolidate low-level state
logic in `Vernacstate`, removing `States` which is now a stub.

There is hope to clean up some stuff regarding the handling of
low-level proof state, by moving both `Evarutil.meta_counter` and
`Evd.evar_counter_summary` into the proof state itself [obligations
state is taken care in #11836] , but this will take some time.

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