[Rust] Support stable Rust

[asfimport]

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

• Specialization

Reporter: Andy Grove / @andygrove

Subtasks:

• [Rust] packed_simd requires nightly
• [Rust] Trait-specialization requires nightly
• [Rust] Remove specialisation from Rust parquet
• [Rust] Migrate CI tests to stable rust
• [Rust] Update docs in regard to stable rust

Related issues:

• [Rust] Remove specialisation from Rust parquet (is related to)
• [Rust] Minimize unstable feature usage (is related to)

_{Note: This issue was originally created as ARROW-6717. Please see the migration documentation for further details.}

[asfimport]

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

[asfimport]

Qingyou Meng / @mqy:
The latest rustfmt available on my local machine is:

$ rustfmt +nightly --version

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

[asfimport]

Andrew Lamb / @alamb:
Migrated to github: apache/arrow-rs#214