feat: merge and sort imports

[asterite]

Description

Problem

Manually arranging imports is time consuming.

Summary

Added two new configurations, reorder_imports (true by default) and imports_granularity (Preserve by default), similar to the ones found in rustfmt. With the default configuration imports are reorganized alphabetically but not merged.

While implementing this I bumped into one code in the test programs like this:

use foo;
use foo::bar;

Merging those in Rust ends up like this:

use foo::{bar, self};

We don't have self in imports, so I had two choices:

1. Try to leave the above unmodified.
2. Implement self in imports.

It turned out that implement self in imports is very easy: when we desugar imports, if the last segment is "self" we just remove it. Then use foo::self is desugared into use foo.

Additional Context

None.

Documentation

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[TomAFrench]

I don't think we should be so aggressive on merging different use statements together. Other changes look good though.

[jfecher]

Code LGTM.

I could see an argument for it either way, but maybe the option should be off by default like Tom said.