Optional Arguments

[noordahx]

Draft PR:

• Resolved merge conflicts of original PR
• Tests failed at OptionalArgs.mls

fun f1(a: Int, b: Int) = a + b
//│ ╔══[ERROR] Type mismatch in operator application:
//│ ║  l.17: 	fun f1(a: Int, b: Int) = a + b
//│ ║        	                         ^^^^^
//│ ╟── type `()` is not an instance of type `Int`
//│ ║  l.17: 	fun f1(a: Int, b: Int) = a + b
//│ ║        	          ^^^
//│ ╟── but it flows into reference with expected type `Int`
//│ ║  l.17: 	fun f1(a: Int, b: Int) = a + b
//│ ╙──      	                         ^
//│ TEST CASE FAILURE: There was an unexpected type error
//│ fun f1: (a: Int | (), b: Int | ()) -> (Int | error)