Define an ExpressionTransform trait

[scovich]

What changes are proposed in this pull request?

Similar to the existing SchemaTransform trait, an ExpressionTransform trait can make it a lot easier to recursively manipulate expressions. Define the trait and introduce an expression depth checker to test it.

This PR affects the following public APIs

Because enum tuple variants cannot be passed as function arguments, we factor out UnaryExpression, BinaryExpression and VariadicExpression structs which the corresponding expression variants can then use. This requires updating match arms throughout the code base.

How was this change tested?

New unit test.

[scovich]

NOTE: While it would be tempting to do Box<BinaryExpression> instead of two boxes here, that would prevent match arms from unpacking it. So we keep the boxes inside the struct, and out of the way.

[codecov]

Codecov Report

Attention: Patch coverage is 73.04348% with 62 lines in your changes missing coverage. Please review.

Project coverage is 80.61%. Comparing base (ac8dcdc) to head (6bc9876).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
kernel/src/expressions/mod.rs	72.76%	52 Missing and 6 partials ⚠️
ffi/src/expressions/kernel.rs	0.00%	3 Missing ⚠️
kernel/src/engine/arrow_expression.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #530      +/-   ##
==========================================
- Coverage   80.76%   80.61%   -0.15%     
==========================================
  Files          67       67              
  Lines       14076    14278     +202     
  Branches    14076    14278     +202     
==========================================
+ Hits        11368    11510     +142     
- Misses       2134     2188      +54     
- Partials      574      580       +6     

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[nicklan]

lgtm!

[zachschuermann]

LGTM just left a quick question and perhaps we want to consider moving much of the new transform code to a new module?

[zachschuermann]

out of curiosity, why usize instead of u64 etc.? I always associate usize as the type for indexing into memory, and feel like I would have just defaulted to u64 here (or even something smaller)?

[scovich]

Not sure about rust, but in c++ size_t (usize equivalent) is the general purpose count/size type.
For example, Iterator::count returns usize.

[zachschuermann]

thanks!