Mullein M2

Copperbox revision 1108.

Initial work on a new version of Mullein, provisionally called M2 - I'll give it a distinct new name once I think of a good one.

I don't really want work on the old code and still call it Mullein as a lot of code needs to change and I always find it dispiriting working with a sandbox where much of the code is broken. The syntax tree needs significant changes to accommodate triplets and duplets, plus I don't like the way I previously handled updating traversals on the old tree.