[Feature] Implement `Func<P..., T>` generic function type and auto function type inference

[Luna-Klatzer]

Is there an existing proposal for this?

• [X] I have searched the existing issues

This feature does not exist in the latest version

• [X] I am using the latest version

Proposal

As with #494 and #499, Kipper aims to support built-in generics and auto-inferred types, which should allow for more complex operations in the language.

This issue more specifically aims to target the topic of a generic function type, which is necessary for saving functions as values in variables and similar. So for example:

def fun(arg1: num) ->  str {
  // ...
}

// Auto-inferred type when referencing the value
fun; // -> Func<num, str>

//  Assignability of a function to a variable
var x: Func<num, str> = fun; // -> Func<num, str>

// Calling the function from the variable
x(1); // -> str

Exact behaviour / changes you want

• [x] #494 (Required for this issue)
• [ ] Internal generic function type which takes any amount of function parameter arguments and takes one return value.
• [ ] High-Level Func<P..., T> generic type, which the user can use to specify a generic function.
• [ ] Integration with existing function call functionality.

[Luna-Klatzer]

Implemented by #673.