Renames variables sizeof_Array and other array_* variables
that were actually intended for slices and not arrays.

Change-Id: I391b95880cc77cabb8472efe694b7dd19545f31a
Reviewed-on: https://go-review.googlesource.com/c/go/+/180919Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>