Move schema and expression FFI code to separate module

[OussamaSaoudi-db]

This PR addresses points 1 & 2 in #358 by moving the schema visitor to the schema.rs module, and moving the KernelExpressionVisitor to the module expressions.rs.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 220 lines in your changes missing coverage. Please review.

Project coverage is 74.69%. Comparing base (da206ed) to head (a722343).
Report is 31 commits behind head on main.

Files with missing lines	Patch %	Lines
ffi/src/expressions.rs	0.00%	164 Missing ⚠️
ffi/src/schema.rs	0.00%	56 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #360      +/-   ##
==========================================
- Coverage   74.71%   74.69%   -0.03%     
==========================================
  Files          43       45       +2     
  Lines        8361     8361              
  Branches     8361     8361              
==========================================
- Hits         6247     6245       -2     
- Misses       1727     1729       +2     
  Partials      387      387              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[nicklan]

lgtm! thanks

[scovich]

Nice!

Eventually we may also want to move the engine bits to their own file, but that's probably not necessary until we add more complete engine support (e.g. engine data, expressions, etc).

[scovich]

aside: Right now, this is only used by the expression visitors.
Should we move it there? Or do we anticipate it being useful more generally?

[OussamaSaoudi-db]

I think this would be useful in any Engine -> Kernel conversions, like how its used for expressions. Right now, I'm not sure if we'd have usecases where the engine must convert anything besides expressions. Perhaps schemas or some other metadata?

[zachschuermann]

lgtm and flagging this is breaking API change if we shuffle modules/etc. around (at least in rust it would be if e.g. it leads to import path changes)