This PR is a suggested design for automatic function calling in Go.
There is no implementation yet.

- Add a Function field to FunctionDeclaration. Populating this
  field enables the function to be automatically called. If you
  set the field manually, you must set schema manually as well.

- Add NewCallableFunctionDeclaration. This takes a function and
  both infers its schema and populates the entire FunctionDefinition.

This design supports three use cases:

1. Manual function calling: set schema but not function.
2. Auto function calling, user schema: set schema and function.
3. Auto function calling, inferred schema: call NewCallableFunctionDeclaration

But it doesn't support:

4. Manual function calling, inferred schema

except in the roundabout way of using NewCallableFunctionDeclaration
and then setting the Function field to nil.