Ion-backed datamodel backend, with annotations preservation

[vgapeyev]

Supersedes / implements Issue #1093

With supporting annotations in Ion data, as requested in Issue #1093, there is tension between importance of annotations in Ion vs annotations being Ion-specific and therefore risking disruption of the established PartiQL data model.

The idea is to pursue a solution where the PartiQL data model is not extended to incorporate annotations, but, instead, Ion annotations are supported only within Ion-sourced data and not within data coming from other sources or not necessarily within constructors that combine Ion fragments into a query result.

This idea can be carried out as part of redesigning ExprValue, as outlined in Issue #1188.