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ARROW-10938: [Rust] upgrade dependency "flatbuffers" to 0.8

[flatbuffers](https://crates.io/crates/flatbuffers) 0.8.0 was released on Dec 10, 2020, with some notable changes:

- new verifier
- common rust traits to FlatBufferBuilder
- new VectorIter
- add FlatBufferBuilder::force_defaults API
- Optional Scalars
- up to 2018 edition
- possible performance speedup
- ... and minor breaking change to some APIs, for example: remove "get_", return Result.

flatbuffers 0.8.0 requires the latest flatc, the git commit for flatc is updated too.

I deliberately commit all changes step by step to make them clear.

Closes #8936 from mqy/flatbuffers-0.8.0

Authored-by: mqy <meng.qingyou@gmail.com>
Signed-off-by: Andrew Lamb <andrew@nerdnetworks.org>