ARROW-11779: [Rust] make alloc module public

[ritchie46]

Polars uses the arrow::memory module. With the backwards incompatible change of #9495, the API is refactored to arrow::alloc.

By making alloc public, users can shift to the new changes.

[nevi-me]

Probably an omission, the intention was to keep alloc public as memory was.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-11779

[alamb]

@ritchie46 I am curious what your use case for using the allocator module directly is? It seems like the main public interface for Arrow aspires to a higher level (Array et al).

[ritchie46]

I use an wrapper type that is basically a Rust Vec combined with an arrow alloctor. AlignedVec. Because this is typed and has the same API as Vec I found this easier to work with.

It is used for instance in FromIterator to build a ChunkedArray (a wrapper around an arrow array) when I know I don't have null types.

When I don't have null values or I can just use an existing null buffer the AlignedVec is more performant for my case.

[alamb]

Thanks @ritchie46 -- the idea of AlignedVec is quite cool (something like that would likely be useful for us in IOx when we also want to go back / forth between Vec and arrow Arrays.

I wonder if it would make sense to consider move AlignedVec and ChunkedArray into the arrow crate itself. 🤔

[ritchie46]

I do think that an AlignedVec in Arrow could speed up some code and make some internal code easier to maintain. If you want I could clean it up and propose a PR.

To my knowledge, the ChunkedArray's are already existent in the C++ implementation. I don't know if the ChunkedArray in Polars is similar to that implementation.

[alamb]

I do think that an AlignedVec in Arrow could speed up some code and make some internal code easier to maintain. If you want I could clean it up and propose a PR.

@ritchie46 I would enjoy helping review that code as well as ChunkedAray

I spent some time reviewing [`ChunkedArray])(https://github.com/ritchie46/polars/blob/6f2dfd43dbf7ab44cbd2904af57d3441a1b52853/polars/polars-core/src/chunked_array/mod.rs#L141-L157) and it looks very useful for something in IOx (the ingest path).

Thank you for the pointers (and the well commented code)