split printer.go into two files; it has become too large:

- nodes.go implements ast node formatting
- printer.go implements the core printing functionality
  and public interface

No code changes except for updating the import clauses
and adding a top-level comment to nodes.go.

R=rsc
http://go/go-review/1016026

src/pkg/go/printer/Makefile

@@ -7,5 +7,6 @@ include $(GOROOT)/src/Make.$(GOARCH)
 TARG=go/printer
 GOFILES=\
 	printer.go\
+	nodes.go\
 
 include $(GOROOT)/src/Make.pkg