Type-specific collections and routing filters

[Vaguery]

Based on #175:

Consider also revising :vectorizable to construct items in this same way. That is, rather than there being an independent vector-of-thing type for each type, consider making a :vector have a :filter attribute which bumps things pushed to it over to be re-routed.

This may also be feasible for :set and some other collection types.

The tricky thing would be working with sub-types (as with :matrix sizes). That is, :vector-concatenate would need to have a specific rule for handling type consistency, or would need to be careful to select arguments.