156 grammar support optional unwrapping in math expr

[mrodz]

• Refactor the optionals codebase
• Update requirements for ?=
  • The name must already be in scope, and have an optional type.

    a: int? = nil
    if a ?= meaning_of_life() {
    # typeof(a) == "int"
    print a
    }
    # typeof(a) == "int?"
    print a

  • Updated unit tests accordingly
• Optional unwrapping with or requires parenthesis for multi-postfix operations (for now)

  get (liberty()) or "death"

• Infer type of nil in specific cases

  1. guns: int? = nil
     get test or "roses"
     #           ^ `str` is not valid here, `int` is expected

  2. get nil or nil
     #          ^ `nil` cannot be here, because `nil` does not satisfy an indeterminate typecheck

  3. get nil or 5 # valid