[dev.fuzz] internal/fuzz: move CoordinateFuzzing args into struct type

[dev.fuzz] internal/fuzz: move CoordinateFuzzing args into struct type

This improves readability a bit, and it should help with compatibility
for future clients when arguments are added or reordered.

Unfortunately, testing still can't import internal/fuzz, so the
interface there can't use this type.

Change-Id: I4cda2347884defcbbfc2bd01ab5b4a901d91549c
Reviewed-on: https://go-review.googlesource.com/c/go/+/308192
Trust: Jay Conrod <jayconrod@google.com>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Katie Hockman <katie@golang.org>