should isolate all rump specific changes to rumprun files

[eyberg]

not sure how possible this is in all cases but if we can we should

[eyberg]

on this note - it would be good to keep a list of substantial diffs for any porting to the next language version bump - for instance there are large amounts of places where we need to add build tags - sometimes it's a case of adding it to new files, sometimes it's a case of moving functionality from old files we created to new ones but that's orthogonal to the places where we made Rumprun specific changes