go/types: fix scope printing (debugging support)

Printing of scopes was horribly wrong: If a scope contained
no object declarations, it would abort printing even if the
scope had children scopes. Also, the line breaks were not
inserted consistently. The actual test case (ExampleScope)
was incorrect as well.

Fixed and simplified printing, and adjusted example which
tests the printing output.

Change-Id: If21c1d4ad71b15a517d4a54da16de5e6228eb4b5
Reviewed-on: https://go-review.googlesource.com/135115


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>