Flatten BoundKind into BoundExpression

[mhk197]

Collapses BoundExpression { kind: BoundKind, dtype } into a single enum, with dtype on each variant.

pub enum BoundExpression {
    Scalar { dtype: DType, scalar_fn: ScalarFnRef, children: Arc<Vec<BoundExpression>> },
    Root { dtype: DType },
}

This is a prerequisite for lambdas, which have no dtype but should be a variant.

[codspeed-hq]

Merging this PR will degrade performance by 13.18%

❌ 2 regressed benchmarks
✅ 1927 untouched benchmarks
⏩ 51 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	fsl_large	204.5 µs	240.1 µs	-14.82%
❌	Simulation	take_map[(0.1, 0.5)]	2.2 ms	2.5 ms	-11.52%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing mk/bound-expression-enum (9296dac) with mk/root-expression (996a4e3)²}

Footnotes

1. 51 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on mk/root-expression (8bd1b1f) during the generation of this report, so 1fd8422 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩