feat: Switch to use Rust stable by default

[sunchao]

Which issue does this PR close?

Closes #.

Rationale for this change

There is little reason for Comet to use nightly at the moment. The only feature we needed from nightly is specialization which is used in a few places that are no longer required.

This PR removes a few stale structs: vector and mutable_vector, which were added during an experiment of implementing Comet's own vector to be used by execution. However, as we've fully moved to use DataFusion, these are no longer needed.

With these structs gone, we can now use stable as the default Rust toolchain for the project.

What changes are included in this PR?

How are these changes tested?

[vaibhawvipul]

Hey @sunchao , good work! You will probably need to update the following too - https://github.com/apache/datafusion-comet/blob/17614c0d6493bdab984e8d04d7c9e92b569539d2/.github/actions/setup-builder/action.yaml#L21

[andygrove]

I branched from this and created https://github.com/apache/datafusion-comet/pull/505, so closing this one. Thanks @sunchao for getting this started.