Add native functions for basic file IO and math calculation

[lifuhuang]

This pull request introduced two basic functions for file read and write. The goal here is to provide a simple way to support long-term memory. I added an example where I leveraged these functions to implement a simple AI memo. But I tried to keep the native functions general enough because they have potentials to be leveraged in other more advanced scenarios as well (e.g., scratchpad for assistant execution planning).

[lifuhuang]

Update: when using the memo feature I implemented with the IO operations in my own fork, I noticed that LLMs were less helpful with numerical inquiries (e.g. "how much milk did I feed the baby today?"), so I pushed a new iteration with a basic calculator function and examples.

Now the assistant is able to do something like this:

User:
How much milk did the baby drink today?

Assistant:
I found the following entries in the memo regarding how much the baby drank today:

• 130 ml at 00:30
• 125 ml at 05:00
• 110 ml at 13:00
• 125 ml at 06:00
• 130 ml at 21:00

I added these amounts together:

130 + 125 + 110 + 125 + 130 = 620 ml.

That's how I reached the total of 620 milliliters.

Sorry if I am hijacking the same PR for two different things and should have sent them in separate PRs :P