Move variable declarations closer to their first uses.

Use an additional string variable s0 for error reporting that references
the original input string. This allows the variable s to be modified.

Change-Id: I4725152490ca1dc10c1161ad8ad2f4ae8933493f
Reviewed-on: https://go-review.googlesource.com/55138Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>