Publically export Arrow crate from datafusion

[alamb]

As suggested by @andygrove and @returnString on #33 (comment)

Rationale:

Make it easy for crates that depend on DataFusion to get the correct version of the arrow-rs crate without having to manually keep versions in check

Proposed Changes:

DataFusion should publically re-export the Arrow crate so that dependent crates automatically get the same Arrow version

[returnString]

I'll take this later today if no-one else wants to claim it immediately :)

[jorgecarleitao]

I though that dependencies could only be declared in Cargo.toml. Or is the idea that we use pub use arrow::* in datafusion/arrow.rs, and ask people to use use datafusion::arrow::X instead of arrow::X in their crates?

[returnString]

I though that dependencies could only be declared in Cargo.toml. Or is the idea that we use pub use arrow::* in datafusion/arrow.rs, and ask people to use use datafusion::arrow::X instead of arrow::X in their crates?

Yeah, my understanding here is we'd use the latter approach of explicitly reexporting arrow. It'd still be possible for people to declare a dependency on arrow and use it directly, but the datafusion::arrow reexport would allow for people to get an appropriate version of arrow by default, rather than having to manually upgrade deps in lockstep :)

[jorgecarleitao]

Makes a lot of sense to me 👍

I think we will need to remember to bump datafusion's major whenever we bump arrow's major in Cargo.toml, as datafusions' public API will now contain arrows' public API (right?)

[alamb]

Thank you @returnString !