Enable repr to work on any tree

By enforcing a canonical signature for __repr__ we enable any mix of Node with other things.
It previously asked for a Node whose children are all Node themselves.
Name all node created

Without name, it is impossible to distinguish between the internal nodes generated by grammar subexpressions, like parenthesis (e), star e*, plus e+, question mark e?, etc.

This is possible thanks to 2d0101e7c20b5de426f7904ea91e7035aec63b0b
2to3
Add epsilon, useful ?
Rewrite ref resolving +check recursive +fix alias
Improve error message
When failing, provide with the incomplete parsetree