Add "only when"

[ReedOei]

Transformers often have certain preconditions that need to be enforced. For this, it is convenient to have only when statements, such as:

only when x >= y

This is actually not difficult to implement, because of how flows work (specifically, how they fail when the desired values cannot be selected). So we can implement this by expanding it into:

x --[ y ]-> x

Similarly, we can expand a precondition like (where sender : address and owner : address):

only when sender = owner

into

sender --[ owner ]-> sender

Finally, we can expand compound preconditions like

only when P and Q

into

only when P
only when Q

then generating code as before. The full grammar, at least for the initial version, should be (where L is any locator; specifically, a source locator, but we can worry about that later):

Precondition ::= "only" "when" Pred
Pred ::= Pred "and" Pred | L Op L
Op ::= < | > | <= | >= | = | != | in

Ideally, we can also optimize the generate code so that no transfer actually needs to take place, and instead we can simply check the preconditions. This may require some changes to the compiler to make things work out nicely, but it is also likely that such changes will make implementing other features (such as consume) work out more nicely.