Implement a way to call a rust function from ketos

[porky11]

(isn't there already? I didn't find it) This would make it easier to call rust functions, which is especially useful, when they do things, which cannot be done in ketos

[murarth]

There are a few ways of creating a Ketos Value that wraps a Rust function, depending on the type of function you have:

• Value::new_foreign_fn wraps a Rust function with the signature fn(&Context, &mut [Value]) -> Result<Value, Error>, the way built-in functions (+, -, etc.) are implemented.
• The macro ketos_fn! wraps any Rust function, so long as its arguments implement FromValueRef and its return type is Result<T, Error> where T: Into<Value>.
• Create a Value of the Function variant, which is like the first option, but avoids a dynamic dispatch call through the ForeignValue trait. This is done transparently when you use a ModuleBuilder to add functions to a new module.

[porky11]

Cool, so even multiple dispatch is supported. I wouldn't have expected functions in the value module