Preserve parentheses in repr for Grammar about parsimonious

[erikrose]

Whoops. I guess I missed a spot when I added parentheses!

[erikrose]

Actually, it's a little trickier than that: parens aren't represented in the final tree by nodes: just by the shape of the tree, as you might imagine. So we'll need to extract them from the shape of the tree itself, unless we want to do icky (but temptingly easy) things like saving the original string of the rule. I don't like saving the string because I like that the repr normalizes the representation and serves as a sanity check that the internal tree is as expected.

I wonder if I could conceptually put parens around everything as I build the repr and then remove any which enclose only a single expression. Maybe pass a "level" var down the as_rule stack or something.