feat(python): ~30% faster iter_rows(named=True) and to_dicts(), if pyarrow available

[alexander-beedie]

This is on top of the previous 10% speedup for to_dicts, but only if pyarrow is available. Speedup varies with dict composition but is always significant (I'm seeing 25-45% gains with a freshly-compiled release build).

Also:

• Repointed to_dicts internally to use iter_rows(named=True).
• Added some Struct coverage to the iter_rows unit test.

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The degree of speedup makes me think I should look at generating the iter-values down in Rust with pyo3 after all; python loops are notoriously slow compared to lower-level languages, and it seems there is some low-hanging fruit here...

[ritchie46]

The degree of speedup makes me think I should look at generating the iter-values down in Rust with pyo3 after all; python loops are notoriously slow compared to lower-level languages, and it seems there is some low-hanging fruit here...

Yeap, we can construct the whole python objects on the rust side with pyo3. We already implemented traits for the conversion so this should be fairly simple.