Freeze and Treeify operations

[goodmami]

Some functions one may want to apply to the graphs work better when the graph can be treated like a tree (e.g. for branch pruning or reliable traversals), so some functions to "freeze" the graph or to reduce it to a tree could be useful. There may be several levels to this:

1. fix edge orientations (e.g. (a :ARG1 b) vs (b :ARG1-of a))
2. fix node relation ordering (e.g. (a :ARG1 b :ARG2 c) vs (a :ARG2 c :ARG1 b))
3. sever reentrancies (e.g. (a :ARG1 (b / abc) :ARG2 b) vs (a :ARG1 (b / abc) :ARG2 (b2 / abc)))