Support for SmallVec in conversion traits

[jakelishman]

Would the PyO3 project be interested in a PR that adds a smallvec feature (akin to hashbrown and indexmap) to add conversion traits for the smallvec crate? In some of the higher-performance places I'm using PyO3 I'm moving some structures to smallvec for better cache locality, and at the moment I need to manually convert whenever one of those objects appears at the Python API boundary.

I'd be happy to contribute the PR, if it's a feature you'd like to merge. I don't know how you feel about expanding the optional trait implementations to every crate under the sun, though!

[davidhewitt]

That'd be fine by me.

[davidhewitt]

Given we have to provide trait implementations here, or thirdparty crates need to provide implementations (or we need to come up with a way to allow downstream implementations to register conversions at runtime, though I think that way lies dragons)...

It's probably more practical for us to maintain the conversions here rather than ask N crates to have PyO3 features (you can imagine they then have to support M language binding features).

So the bar for what crates to accept needs to be pretty high; so far we've done this case-by-case as long as they're relatively core ecosystem crates which have types which map well onto Python types.

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The only cost at the moment of not accepting a crate is that all users downstream just have to wrap in newtypes or use #[pyo3(from_py_with)], so it's not that hard for users to deal with crates not integrated directly with PyO3.