Cleanup chat template API

[vlovich]

1. Make the template not an optional for apply_chat_template. This ensures you don't accidentally use the chatml template.
2. Improve performance for the expected case of using get_chat_template by returning a new LlamaChatTemplate struct that internally stores the string as a CString. Unless you try to explicitly create a copy or print, there's no extra copy into a Rust string that's created. Similarly, get_chat_template -> apply_chat_template no longer copies the template string.
3. Improve documentation including documentating what the add_ass parameter does and suggestions on what values you probably want to use. Additionally, I've made get_chat_template and apply_chat_template docs refer to one another to make it easier to discover how to use this.

This resolves #640 and #641

@MarcusDunn hope you don't mind, but instead of separate &str vs &CStr, I've instead created a new LlamaChatTemplate struct that internally stores the CString with convenience methods to easily retrieve &str/String from it or to construct it from a &str. I found that to be a bit cleaner.