noreturn functions
==================

Syntax 1:
+ short

    func f
        Int x
    noreturn
    code
        ...
    end


Syntax 2, special "NoReturn" type:
* Inspired by Python's type hints, but this would be a real type
* the "NoReturn" type can't be used elsewhere.
- one extra line

    func f
        Int x
    return
        NoReturn
    code
        ...
    end


Syntax 3, special "Never" type:
* Inspired by Python's type hints, but this would be a real type
- one extra line
- the "Never" type can't be used elsewhere, so it could be confusing.

    func f
        Int x
    return
        Never
    code
        ...
    end