Unified construct for forward/reverse branching

[dmlloyd]

Right now you have to use different constructs for forward versus backward branches. It's easy enough to write a marker class which automatically backfills branches, but putting this in the main API would be a nice feature. Also something that is not really possible is to combine multiple such markers - to do so would require BranchEnds to be filled by a CodeLocation (right now they can only be added as a mark into the CodeAttribute's current position).