Initializes Datum and adds functionality to evaluator

[johnedquinn]

Relevant Issues

• None

Description

• Initializes Datum, the engine's representation of a value
• Also and adds functionality to evaluator. Currently, Datum is all over the evaluator, however, we convert to PartiQLValue when calling UDFs and doing comparisons.
• While working on the JOIN PR (#1438) and PartiQL Cursor PR (#1443), it became apparent to me that our modelling of PartiQLValue is tightly coupled with Java's type semantics. In many places, we rely on Java's types (example:x is StructValue<*>) rather than relying on our plumbing: (x.type == PartiQLValueType.STRUCT). On top of that, as part of the JOIN PR, I found it incredibly difficult (and sub-optimal performance-wise) to coerce nulls of a particular type to a null of another type. This PR takes these learnings and completely overhauls PartiQLValue to a representation that we are all familiar with: Ion's AnyElement. In a way, this modelling is as close as we can get to C's Union type, and by modelling it this way, we gain control over our type system.
• Notice that the API proposed is called PQLValue. I intend for its core to be copied and pasted directly into PartiQLValue. I have also written this in Java, as I've noticed our inability to control our public API by writing in Kotlin. On top of that, PQLValue is written in Java to get us closer to using primitives directly. That being said, PQLValue will NOT exist during the V1 release. Its contents will replace the contents of PartiQLValue -- and PartiQLValue will be written in Java.

Internal Notes

• Notice that I attempt to keep the null-safety that Kotlin provides by marking all applicable API's with @NotNull.
• Similarly, I attempt to use primitives as much as possible -- limiting our memory footprint and maintaining the null-safety.
• I've left remaining action items to fully replace PartiQLValue in the KDocs of PQLValue. These include adding the comparator, equals, hashCode, etc. These should be able to be added fairly quickly.

Other Information

• Updated Unreleased Section in CHANGELOG: NO
• Any backward-incompatible changes? NO -- not really, however, PQLValue intends to replace PartiQLValue
• Any new external dependencies? NO
• Do your changes comply with the Contributing Guidelines and Code Style Guidelines? YES

License Information

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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Additional details and impacted files

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[johnedquinn]

Only change is to fix the test/partiql-tests submodule otherwise ship it.

I believe there is a bug in GitHub's rendering of the submodule diff. For some reason, the commit is the same on v1 as on johnedquinn:v1-eval-value-rebased. Checked the files being "changed" and they're all already in v1. I pushed twice to move to some older submodule commit and then back to what is currently on v1, but it's still not rendering appropriately.