Remove unsafe buffer usage warnings in flat_tree.h

The insert(iter, iter) method is where things go bad if the inputs are
pointers. Add an overload for pointers that makes a span from them and
mark it as UNSAFE_BUFFER_USAGE.

The insert(iter, iter) method then assumes the iterator checks to be
from the same container when comparing, and no UNSAFE_BUFFERS is needed
internally.

Add insert_range() member to insert objects without dealing with
iterator pairs.

Bug: 40284755
Change-Id: I83273ce0c17d8420e09d58520b8f050fb562269c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5847689
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1353599}