First PR cleanup

[connortsui20]

Problem

#4 Was the first major PR, but since we were making a lot of changes last minute there were a few things that slipped through with respect to quality control.

Summary of changes

• Added very strict clippy documentation requirements. Discussion below.
• Added documentation for everything except the operator::relational::physical module and the operator::scalar module, as those seem kind of unstable right now.
• Changed the scalar generic param of expressions to use ScalarGroupId instead of GroupId.
• Added several TODOs that I think will be resolved pretty quickly, though I'd appreciate it if others took a look at those in the case that we can actually get those done now.

Now that we have something to work off of, requiring documentation on everything (including private items) means that we'll pay the cost right now of making sure people understand what we're doing in the future. I'm willing to relax it a little bit if we put other stuff in place that ensure nobody in the future encounters large chunks of code with zero documentation.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 26 lines in your changes missing coverage. Please review.

Project coverage is 0.0%. Comparing base (c41cf41) to head (4806b77).

Files with missing lines	Patch %	Lines
optd-core/src/rules/implementation/hash_join.rs	0.0%	12 Missing ⚠️
optd-core/src/rules/implementation/table_scan.rs	0.0%	8 Missing ⚠️
...d-core/src/rules/implementation/physical_filter.rs	0.0%	6 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
...ore/src/rules/transformation/join_associativity.rs	0.0% <ø> (ø)
...ore/src/rules/transformation/join_commutativity.rs	0.0% <ø> (ø)
...d-core/src/rules/implementation/physical_filter.rs	0.0% <0.0%> (ø)
optd-core/src/rules/implementation/table_scan.rs	0.0% <0.0%> (ø)
optd-core/src/rules/implementation/hash_join.rs	0.0% <0.0%> (ø)

[AlSchlo]

Perhaps we should rename logical scan to "get". Scan semantically sounds more like a physical implementation.

[connortsui20]

Perhaps we should rename logical scan to "get". Scan semantically sounds more like a physical implementation.

If we're going to rename it at all, it should probably be named Read instead of Get, but even so I think Scan is more universal and it will be clear what we're all talking about when we mention it.