Simplifies joins and fixes bugs

[johnedquinn]

Relevant Issues

• None

Description

• Increases conformance by 4 tests (.07%). See report.
• Simplifies joins by using Kotlin's helpful iterator inline function. This substantially simplifies our code at the cost of some performance. The reason I'm making this move is that there are bugs in the current implementation, and for the sake of our goal (conformance, not performance), I'm moving towards a simpler implementation. Once we pass all join tests, we can re-evaluate our implementation for performance reasons.
• This PR now passes 8 additional tests, however, there are now 4 failing that were not previously failing. This is due to our inability to correctly coerce nulls to other types. Please disregard this issue as this is being handled in #1451.

Other Information

• Updated Unreleased Section in CHANGELOG: NO
• Any backward-incompatible changes? Maybe?
  • There is one change that is worth noting. <struct evaluating to null>.a returns null. Is this right? IMO, it makes the most sense, and it makes the modelling of JOINs very very easy.
• 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:

Please upload report for BASE (v1@5b86afc). Learn more about missing BASE report.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## v1 #1438 +/- ## ===================================== Coverage ? 13.70% Complexity ? 31 ===================================== Files ? 29 Lines ? 1729 Branches ? 190 ===================================== Hits ? 237 Misses ? 1470 Partials ? 22 ``` | [Flag](https://app.codecov.io/gh/partiql/partiql-lang-kotlin/pull/1438/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=partiql) | Coverage Δ | | |---|---|---| | [CLI](https://app.codecov.io/gh/partiql/partiql-lang-kotlin/pull/1438/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=partiql) | `13.70% <ø> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=partiql#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[RCHowell]

Why draft?

[johnedquinn]

Why draft?

Currently incorporating the change for check() that is causing some of the conformance tests for joins to fail.