if assign / guard
=================

Swift has "if let", which makes the identifier accessible only inside
the "if" block, which is only entered if the value is not null.

Similarly, there is a "guard let", but in that case the value is
accessible only in the statements following the "guard".

Improvements
------------
"if let" is has a fairly obvious meaning, but type inference does not belong
in SLUL, so it should not be copied as-is.

"gaurd let", on the other hand, is in my opinion NOT very obvious, and you
have to read the manual. SLUL should strive to be as obvious as possible!


Syntax 1
--------

    if Thing t = get_thing() {
        t.do_stuff()
    }

    require Thing u = get_thing() else {
        ...
    }
    u.do_stuff()

Problems:
- The else might not be visible if the line is long!
- Multiple things may need to be tested!


Syntax 2
--------

    Thing t = maybe get_thing() else goto fail
    Thing u = maybe t.other_thing() else goto fail

Problems:
- What if multiple things can go wrong?
    Thing t = maybe get_thing().other_thing(a / possibly_zero) else goto fail