Add an EstimatedStateSize to some IEmulatorService

int EstimatedStateSize { get; }

Returns the estimated size, in bytes, of a single savestate. Should be callable at any time with a loaded core. Should be a constant size for any particular instance of a core. Can be a different size for different instances of the core (different games, etc.). Should reflect the size of a single state, but no strong requirement is put on how accurate that is. Should tend towards worst case rather than best case scenarios, overestimating if needed.

This could be used by rewinders, state managers, and such, to build settings

Start by locating IEmulatorService and its implementations in the BizHawk source, then determine which services need the EstimatedStateSize property. Check how savestates are represented and consumed by rewinders or state managers. Done means applicable loaded-core services expose a stable, conservative byte estimate that can be queried at any time.