ensure that every integer conversion fits into the destination
type, and for any unchecked ones, annotate why they are safe.
additionally, any integers we pass into slice headers need to
check that they are not negative.
all of our allocations should check for allocation failure and
return an error if there was a problem. even though an allocation
just failed, we don't pre-allocate the failure because we don't
want the callers to free it like they have to with the other
errors we return. maybe we should just panic.
Change-Id: Id4dfad802d312d35e565041efc9872453c90d793
