Clean up dependencies of ebuild

[dongsupark]

The Gentoo ebuild file has a long list of dependency crates, which includes some unnecessary libs as well.
Let's investigate how the list can be optimized.

• Enable only necessary feature flags of tokio in Cargo.toml : Cargo: optimize feature flags of tokio #20
• Clean up unnecessary crates from Cargo.lock, e.g. windows libs.

[dongsupark]

It is actually 2 different issues: optimization of feature flags in Cargo.toml (done in #20), and actually deleting unnecessary libs from Cargo.lock.

The latter is more tricky to resolve, because cargo-ebuild simply copies over all deps in Cargo.lock to an output ebuild, but rust cargo almost always assumes all libs are listed in Cargo.lock, including windows libs.
Some folks recommend calling cargo -Z avoid-dev-deps (only available in nightly), but that still does not bring any difference.

See also related discussions: rust-lang/cargo#10801, rust-lang/cargo#5133.

[dongsupark]

Let's close, as there is no solution for the Cargo.lock issue.
Feel free to reopen if there is a way forwards.