archive/tar: move parse/format functionality into strconv.go

archive/tar: move parse/format functionality into strconv.go

Move all parse/format related functionality into strconv.go
and thoroughly test them. This also reduces the amount of noise
inside reader.go and writer.go.

There was zero functionality change other than moving code around.

Change-Id: I3bc288d10c20ebb3814b30b75d8acd7be62b85d7
Reviewed-on: https://go-review.googlesource.com/28470
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>