Survey the `security` submodule

This contains some of the earliest work on typed headers, and precedes the cohesive design push we've initiated since. This makes it likely that the work there doesn't conform to the shape of the other headers, and we may need to rewrite this.

For the next major I'd like to propose we survey the work done, and either change it or remove it before the next major so that we can introduce "correct" versions without the need for an additional major change.