ARROW-4137: [Rust] Move parquet code into a separate crate

[sunchao]

This moves the parquet related code into a separate sub-crate that
depends on arrow.

[kou]

I've added a commit to update version on release process.

[andygrove]

LGTM

[sunchao]

Thanks @kou ! Regarding the release, I'm wondering whether every crate (e.g., arrow, parquet) has to use the same version number, and be released at the same time. Could we do separate releases for the crates with different version numbers?

[kou]

whether every crate (e.g., arrow, parquet) has to use the same version number

We use different versions in C++. Parquet C++ uses https://github.com/apache/arrow/blob/master/cpp/src/parquet/.parquetcppversion and other C++ modules (Arrow, Plasma and Gandiva) use the same versions.

We use the same versions in C and Ruby.

I think that we can use the different versions in Rust too but we may need to discuss it on mailing list.

Could we do separate releases for the crates with different version numbers?

I'm not sure separate release is good for us.

We use different versions in C++ but we don't use separate release for C++.

We use separate release for JavaScript.

Separate release will be possible but we may need to discuss it on mailing list.
Our release cycle will be shorter in 2019. So we may not need separate release.

[kou]

BTW, we use subdirectories both for Arrow and Parquet in C++, C and Ruby:

• https://github.com/apache/arrow/tree/master/cpp/src {arrow/,parquet/}
• https://github.com/apache/arrow/tree/master/c_glib {arrow-glib/,parquet-glib/}
• https://github.com/apache/arrow/tree/master/ruby {red-arrow/,red-parquet/}

We may like the structure in Rust. (I think that this is out of scope of this pull request.)

[sunchao]

Thanks @kou ! good suggestion on the sub-directory - will look into that after this PR. I'll also send an email to the dev mailing list.

[kou]

Thanks!

[kszucs]

Agree with @kou on the dub-directories.

[kszucs]

Thanks @sunchao for the PR and thanks @andygrove @paddyhoran for the reviews!