The previous version of all the node.String methods printed the parse
tree and was useful for developing the parse tree code. Now that that's done,
we might as well print the nodes using the standard template syntax.
It's much easier to read and makes error reporting look more natural.

Helps issue 2644.

R=rsc, n13m3y3r
CC=golang-dev
https://golang.org/cl/5553066