[V1] Replace PartiQLValue with Datum

[RCHowell]

NON-BLOCKING for rc.3

• Datum was designed as a low-level API for evaluation.
• PartiQLValue was designed as a DOM for PartiQL Values

This issue is addressed once the following questions are answered (and implemented) .. even if obvious.

• Which value representation is used in the AST, planner, evaluator, other.?
• Which representation has readers/writers?
• Datum can efficiently wrap PartiQLValue.
• How is the variant type modeled in Datum and PartiQLValue.
• [V1] Complete datetime and interval types #1657
• Remove PartiQLValue usage in AST (Alan)
• Remove PartiQLValue usage in Plan (John? I think we can internalize)

[RCHowell]

A more detailed doc is in the works.

Answered

• AST has own rep, plan rep undecided, datum everywhere else.
• Datum has reader/writers
• Datum is PartiQL value but with fat interface and internalized impls
• PRs Generalizes back-tick Ion to a variant pair in the AST #1591 and Define datum reader/writer interfaces with variant #1635 show variant modeling across the stack