[Feature] Implement function calls from function references (variable-bound functions)

[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

Implement function calls from function references (variable-bound functions), which allow the user to store functions and then call them up again when needed.

For example:

• Storing scope-bound functions:

  def add(a: num, b: num) -> num {
  return a + b;
  }
  
  const locAdd = add;
  locAdd(1, 2); // -> 3

• Storing lambda functions:

  const subt = (a: num, b: num): num -> a - b;
  subt(1, 2); // -> -1

Exact behaviour / changes you want

• [x] #584
• [x] Implement proper types for defined functions and lambdas, so their references can be stored and called up with proper typing.

[Luna-Klatzer]

Implemented by #681.