Chapter 10: Functions

[jeschkies]

This patch implements the chapter on functions.

Instead of using an interface LoxCallable and and anonymous instance for native functions
I opted for an enum with the variants Native and User. Native functions are defined with a
function pointer and user functions are implemented the same way LoxFunctions are
implemented in the book.

The biggest difference is that we use Err and Result to jump back out of a call stack as
we don't have Exceptions.