Fix WB4 implementation

[kwantam]

Current WB3/WB4 impl is out of spec:

So my interpretation of WB4 was wrong. We have to apply it in order of precedence. Thus, we cannot transform the input before attempting to apply WB3 (and the previous rules). Only then do we transform, and at that point the previous rules can no longer be applied again.

[Manishearth]

The reason this is tricky to implement is that the processing model in the spec is different than a simple iterator.

The spec basically says to go down the list, applying each rule in order. When applying a rule, you apply it simultaneously everywhere.

So we can't just apply WB3 and WB4 in order of precedence whilst iterating. During forward iteration, whilst applying WB4 we need to repeatedly check if WB3 would not apply, and reenter the loop if so. This gets more complicated during reverse iteration.