This change adds missing docs, collapses single-line import paths,
removes unsed method placeholders and renames str.go to strconv.go.

Change-Id: I2d155c838935cd8427abd142a462ff4c56829703
Reviewed-on: https://go-review.googlesource.com/37814
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>