[Rust] Generate flatbuffers code automatically

[asfimport]

This depends on the following upstream flatbuffers issues:

• [Rust] Generated code is inconsistent for fields named type google/flatbuffers#5052

• [Rust] Generated code produces warnings about snake case / camel case google/flatbuffers#5053

• [Rust] Provide option to override default package / file names google/flatbuffers#5054

  Once they are resolved We should generate flatbuffers code automatically,
  as suggested by [~nevi_me]:

  // [arrow/rust/arrow/build.rs]
  
  use std::path::Path;
  use flatc_rust;
  
  fn main() {
      flatc_rust::run(flatc_rust::Args {
          lang: "rust",
          inputs: &[
              Path::new("../../format/FIle.fbs"),
              Path::new("../../format/Message.fbs"),
              Path::new("../../format/Schema.fbs"),
              Path::new("../../format/Tensor.fbs"),
              Path::new("../../format/SparseTensor.fbs"),
          ],
          out_dir: Path::new("./src/ipc/gen/"),
          // doesn't seem to be honoured
          includes: &[Path::new("../../format/")],
          ..Default::default()
      }).expect("Unable to build flatbuffer files");
  }
  
  // [arrow/rust/arrow/Cargo.toml]
  [package]
  ...
  build = "build.rs"
  
  ...
  
  [build-dependencies]
  flatc-rust = "0.1"

Reporter: Krisztian Szucs / @kszucs

_{Note: This issue was originally created as ARROW-5135. Please see the migration documentation for further details.}

[asfimport]

Andrew Lamb / @alamb:
Migrated to github: apache/arrow-rs#42