Alternative data backends

[vgapeyev]

Alter the PartiQL specification to reflect re-imagining the PartiQL data model as a collection of data construction and deconstruction requirements, as outlined in Issue partiql-lang-kotlin/#1188.
It is probably better to do most of this specification work after gaining the implementation experience.

As part of the re-formulation, PartiQL will impose a new requirement of guaranteeing preservation of internal representations of values under specific circumstances. (Such as, that if an Ion fragment has not been destructed/inspected by a query, this fragment should have its structure intact — including presence of annotations — when the fragment is used to construct a larger result.)