Specify semantics of #ifTrue:/#ifFalse: and allow/disallow Smalltalk 80 style optimizations

[smarr]

Often Smalltalk implementations would special-case control structures to simplify implementations.

Personally, I think this isn't great. Though, I haven't bothered implementing it correctly either.

It would be good to specify the semantics, or, at the very least, explicitly have an example, and tests to ensure that #ifTrue: and similar are treated as pure messages, and any implementation optimization is not observable from the language itself.

See smarr/SOMpp#7 for further discussion.

@ltratt's example should work. I don't think there's any good justification for it not to work.

T = (
    ifTrue: x = ( ^2 )

    run = (
        (self ifTrue: 3) println.
    )
)