Should empty nodes be allowed? #10

In principle, it is possible to have nodes of length zero: this would be an internal node without children.

However such nodes are annoying to work with. For example, with empty nodes there maybe arbitrary number of nodes at a given offset (as opposed to at most two for non-empty nodes). So it seems a great idea to forbid this node.

That means that care must be taken to parse stuff like use foo::92; without empty nodes: we know that there should be a path segment after ::, but we shouldn't create a node for it unconditionally.

A funny edge case here is an empty file: We'll have to create a file node for it, and it will be empty.