Rewrite almost everything

The API stays mostly the same, except that using NodeIds is always
unsafe now, and some trait implementations have disappeared. The Default
implementation for Tree didn't seem to make much semantic sense.

On the other hand, there are some new derived trait implementations, and
iterators now implement ExactSizeIterator and DoubleEndedIterator where
possible. IntoValues is replaced with IntoIter and an IntoIterator
implementation.

Author: causal-agent