* Added the ability to save and load individual conversations in a batched executor.
 - New example
 - Added `BatchedExecutor.Load(filepath)` method
 - Added `Conversation.Save(filepath)` method
 - Added new (currently internal) `SaveState`/`LoadState` methods in LLamaContext which can stash some extra binary data in the header

* Added ability to save/load a `Conversation` to an in-memory state, instead of to file.

* Moved the new save/load methods out to an extension class specifically for the batched executor.

* Removed unnecessary spaces

- Re-implmented `Rewind` as an extension method using `Modify` internally
 - Implemented `ShiftLeft`, which shifts everything over except for some starting tokens. This is the same as the `StatelessExecutor` out-of-context handling.
 - Starting batch at epoch 1, this ensures that conversations (starting at zero) are below the current epoch. It also means `0` can always be used as a value guaranteed to be below the current epoch.

Conversations can be "forked", to create a copy of a conversation at a given point. This allows e.g. prompting a conversation with a system prefix just once and then forking it again and again for each individual conversation. Conversations can also be "rewound" to an earlier state.

Added two new examples, demonstrating forking and rewinding.