Support stable Rust

[alamb]

Note: migrated from original JIRA: https://issues.apache.org/jira/browse/ARROW-6717

I'm creating this issue to track all the stories we need to implement to be able to use stable Rust.

• Specialization

[alamb]

Comment from Andrew Lamb(alamb) @ 2020-12-15T21:26:46.102+0000:

Update: As of Dec 2020 Arrow will work with stable Rust by default. If we enable the `simd` features, nightly rust is still required

Comment from Qingyou Meng(mqy) @ 2021-01-06T11:24:12.097+0000:

The latest `rustfmt` available on my local machine is:

```

$ rustfmt +nightly --version

rustfmt 1.4.30-nightly (acd94866 *2020-12-20*)

```

[alamb]

Closing this one in favor of a more specific task (arrow/parquet all work with stable, though the simd feature still requires nightly)