arrow

ARROW-5947: [Rust] [DataFusion] Remove serde crate dependency

I added a dependency to serde_json early on so that I could serialize logical query plans because I wanted a way to pass them between processes. However, this was just a short term hack and is non-standard. I would like to remove this now.

I am now using gRPC in another project and serializing plans that way based on the Gandiva protobuf def. I will start a discussion on the mailing list in the next 1-2 weeks about pushing some changes into the Arrow repo related to this.

Author: Andy Grove <andygrove73@gmail.com>

Closes #4879 from andygrove/ARROW-5947 and squashes the following commits:

4c28ad6ed <Andy Grove> fix conflict