Previously, only the empty interface could be formatted to print on a
single line. This behaviour made short one-method interfaces in function
definitions and type assertions more verbose than they had to be.

For example, the following type assertion:

    if c, ok := v.(interface {
        Close() error
    }); ok {
    }

Can now be formatted as:

    if c, ok := v.(interface{ Close() error }); ok {
    }

Fixes #21952

Change-Id: I896f796c5a30b9f4da2be3fe67cb6fea5871b835
Reviewed-on: https://go-review.googlesource.com/66130
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>