* Add design-at-version resolution

This adds separate singular and plural resolvers for design-at-version,
with tests and references in the design-type.

DesignsAtVersionResolver is nested under Version - This reflects the
fact that this particular resolver requires that it is mounted on a
version

This also importantly ensures that only singular arguments are passed in
singular context, and only plural ones in plural context. Mutually
incompatible arguments are detected and raised as errors.

Equally importantly, this ensures that any objects we fetch by ID have
to pass the same reference checks as the other ways of resolving them,
and they must be coherent with  the implicit version context, and
visible at that version.

Three variants of version resolvers are distinguished:

* when there is no relevant context (e.g. top level):
  - singular resolver able to retrieve versions by ID
* when there is a relevant context (a source of versions)
  - plural resolver to find all versions up-to a given version
  - singular resolver to find a version by ID or SHA

Resolvers are split up into custom singular versions where this makes
code simpler.

Single resolvers return self as single form

This introduces a shared context for efficiency