refactor(rust): Lower arbitrary expressions in the new streaming engine

[orlp]

Most non-elementwise expressions still fall back to the in-memory-map, but at least the groundwork is laid for lowering more and more expressions to the streaming engine over time.

Known bug: common subexpression elimination does not work for the streaming engine in general.

An example:

df.lazy().select(
    gt_avg = pl.col.x.filter(pl.col.x > pl.col.x.mean()).sum(),
    mae = (pl.col.x - pl.col.y).abs().mean()
)

[codecov]

Codecov Report

Attention: Patch coverage is 1.26183% with 313 lines in your changes missing coverage. Please review.

Project coverage is 80.24%. Comparing base (a601ccf) to head (791762b).
Report is 21 commits behind head on main.

Files	Patch %	Lines
...ates/polars-stream/src/physical_plan/lower_expr.rs	0.00%	255 Missing ⚠️
crates/polars-core/src/schema.rs	0.00%	18 Missing ⚠️
crates/polars-stream/src/graph.rs	0.00%	14 Missing ⚠️
crates/polars-stream/src/nodes/multiplexer.rs	0.00%	9 Missing ⚠️
crates/polars-plan/src/plans/options.rs	0.00%	6 Missing ⚠️
crates/polars-stream/src/execute.rs	0.00%	5 Missing ⚠️
crates/polars-stream/src/physical_plan/lower_ir.rs	0.00%	3 Missing ⚠️
crates/polars-stream/src/expression.rs	0.00%	2 Missing ⚠️
crates/polars-stream/src/skeleton.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18315      +/-   ##
==========================================
- Coverage   80.35%   80.24%   -0.11%     
==========================================
  Files        1501     1502       +1     
  Lines      199236   199531     +295     
  Branches     2837     2837              
==========================================
+ Hits       160087   160105      +18     
- Misses      38623    38900     +277     
  Partials      526      526              

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[ritchie46]

Looking an awe at the complexity involved into a seemingly "simple" query. 😄