This isn't strictly necessary now that BIGNUMs are safe, but we get to
rely on type-system annotations from EC_SCALAR. Additionally,
EC_POINT_mul depends on BN_div, while the EC_SCALAR version does not.

Change-Id: I75e6967f3d35aef17278b94862f4e506baff5c23
Reviewed-on: https://boringssl-review.googlesource.com/26424
Reviewed-by: Steven Valdez <svaldez@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>