* 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

* Previously when a conversation was forked this would result in both the parent and the child sharing exactly the same logits. Since sampling is allowed to modify logits this could lead to issues in sampling (e.g. one conversation is sampled and overwrites logits to be all zero, second conversation is sampled and generates nonsense). Fixed this by setting a "forked" flag, logits are copied if this flag is set. Flag is cleared next time the conversation is prompted so this extra copying only happens once after a fork occurs.

* Removed finalizer from `BatchedExecutor`. This class does not directly own any unmanaged resources so it is not necessary.

Replaced `BatchedExecutor.Prompt(string)` method with `BatchedExecutor.Create()` method. This improves the API in two ways:
 - A conversation can be created, without immediately prompting it
 - Other prompting overloads (e.g. prompt with token list) can be used without duplicating all the overloads onto `BatchedExecutor`

Added `BatchSize` property to `LLamaContext`

- 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.