Make nodes use GrinValues for tags.
Add a function render to Pretty,
Rename GrinValue -> Value and join Value + Pattern classes.
class Value => class Literal
Add the concept of node tags.
Move around some Pattern instances.
More case pretty printing issues.

First a scoping bug made case printing wrong.

Also, order was wrong.
Introduce a Core syntax, free from 'Pattern a'
Remove unneeded cleverness from declarations.