Identifier construction: To prefix or not to prefix

On 2022-09-21 we debated how to actually form the identifiers. Like, is there a ``, and/or a ``, and are these modifiers used just for returning identifiers, or are they actually digested, since our protocol involves digesting digests.

Here are some thoughts:
- I think it will be useful to disambiguate the terms and , then: identifier = `:.`
- we should probably include prefixes for the "sequence_digests" array
- the "sequence_digests" array should therefore refer to identifiers, rather than digests, and then probably renamed to "sequence_identifiers"
- we should probably also include prefixes/type_prefixes for the level 1 digest algorithm, but then we have to define these type prefixes for each array.
- it seems like the definition of the type prefixes should happen by an authority at a level higher than our working group.
- are the prefixes actually added before the digest, or just returned to the user? There is not really any identifiability value added in actually *digesting* them. Could they just be affixed at the return/display stage? This makes the specification more universal.
- Would prefixes be required or optional for the *input* from a user, when requesting a lookup given an identifier/digest?