rename to_i32 -> trunc, and add round

This plumbs through round but doesn't use it.  I want that change to be
its own CL.  It's nice to have assembler support and the name changes
even if I revert using round.

Change-Id: I6d67ec5c63546069eb7cc1c91599b599bafcda66
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/253724
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>