Re-implement NLTK tree

[andreasvc]

Would allow a potentially significant speedup for treebank transformations and grammar extraction.

Wishlist:

• represent all treebank information: functions, morphology, lemmas, &c.
• combine indices and words in one datastructure
• parent pointers, sibling pointers
• store yield of each node, i.e., tree.leaves(). modification of tree triggers update in all ancestors.
• automatic canonicalization
• mutable and immutable versions. immutable version could use C arrays/structs.
• perhaps specific optimized version for binary trees.