- Modified `GetEmbeddings` method to be async

FixedSizeQueue:
 - Enqueue would always stop one short of filling the capacity
 - Fill would only _replace_ existing items. It was only used in a place where there were not existing items! Removed the method entirely.

LLamaGrammarElement:
 - Converted into a `record` struct, removed all of the (now unnecessary) equality stuff.