Add fake root to RST-dep tree

It is common practice in dependency parsing to add an artificial root node.
This root node governs the (actual) root(s) of the dependency structure.
It is usually placed at the beginning of the input sequence.

This artificial root node cannot have incoming dependencies, which should solve root-related downstream issues, e.g. in attelo's MST decoder.

We may also want to have these as constants, generally trying not to have magic values in code (I'm pretty guilty of them though… the problem is that sometimes magic values are more readable/evocative than _SOME_CONSTANT_NAME)