Implementer guidance: enum vs. heapless vec vs. size-and-length

[chrysn]

At some point in time we'll probably want to come up with a style guide that helps implementers decide whether the preferred way to do this is to have an enum with algorithms, this, or a heapless Vec -- and which of those are preferred for which type and why.

For the time being, we probably goes with what is easiest at that moment, and collect experience.

This is the place to do that. Considerations so far:

• Unlike slices and vec, algorithms have a niche.
• heapless might deduplicate nicely (but didn't test that, let's not take my word for it)