Rust: builder-based params, per-module errors

[yan-zaretskiy]

Here we further rework the Rust bindings API to be more idiomatic, and also align closer with the underlying C++ namespacing. More specifically:

• Every param type is built with a validating builder instead of new() + chained set_*. Unset fields keep the C-library defaults; out-of-range values are rejected by build().

// before
let params = IndexParams::new()?.set_graph_degree(64).set_intermediate_graph_degree(128);
// after
let params = IndexParams::builder().graph_degree(64).intermediate_graph_degree(128).build()?;

Thus, by the time the params instance is constructed, it is guaranteed to be valid.

• Resources stream is set at construction. Instead of a post-construction setter, we provide a custom stream via unsafe { Resources::with_stream(stream) }. The Resources::new() keeps the default stream.
• Module layout mirrors C++ namespacing. ANN indexes moved under cuvs::neighbors.
• The crate-wide cuvs::Error/cuvs::Result are removed. Each module returns its own thiserror enum, e.g. CagraError or IvfFlatError. This enables individual index errors to be more granular, instead of creating a single error type encompassing all failure modes.

[tgockel]

This isn't a big deal, but might be worth noting -- the exponent p is not always honored. Over in the C code, this is only moved when metric_arg exists. Might be worth a comment...it's not a regression from the existing C code and I'm not sure how to address it without a major refactor.

[tgockel]

You probably don't want to export these native cudaDataType_t, cuvsIvfPqCodebookGen, and cuvsIvfPqListLayout types publicly, since everything else is wrapped.

[tgockel]

This applies to other news in this file, too: Does the C library have a facility to handle validation of these FFI params for you? It might be worth using to avoid duplicating work.

[tgockel]

Very minor, but prefer .unwrap_or_else(|| unsafe { (*params.handle).algo.into() }) so you only deref in the None case. unwrap_or eagerly evaluates even when algo is Some(_).

[tgockel]

Since this is deleted, the example over in the README Rust API that uses IndexParams::new() won't work any more. For Rust, there's a clever trick you can use to make sure your READMEs always compile:

#[cfg(doctest)]
#[doc = include_str!("../../README.md")]
pub struct ReadmeDocTests;

This picks up the triple-ticks and makes sure they compile (https://doc.rust-lang.org/rustdoc/write-documentation/documentation-tests.html). Rust is the assumed language, but the examples currently all specify c/python/rust/etc -- Rust doctest will ignore non-Rust languages.

There are some other things in examples/rust that use this same pattern that fail to compile. CI can catch this for you by running cargo build --examples