Make loom a dev-dependency

[stefnotch]

Loom is currently only being used as a dependency for tests, as far as I can tell. However, it is included using [target.'cfg(loom)'.dependencies.loom].
This comes with a few issues, mainly

• cargo vendor vendors dependencies of loom.
• Dependencies of loom are included in Cargo.lock when using a crate that depends on loom.
• cargo audit includes dependencies of loom when using a crate that depends on loom.

The workaround for these problems is to make loom an optional dependency: crossbeam-rs/crossbeam#666
from > > * cargo vendor vendors dependencies of loom.

• Dependencies of loom are included in Cargo.lock when using a crate that depends on loom.
• cargo audit includes dependencies of loom when using a crate that depends on loom.

The workaround for these problems is to make loom an optional dependency: crossbeam-rs/crossbeam#666

Could this be improved? Either by making loom an optional dependency, or by moving it to dev-dependencies. Currently this is an issue for me.

[nagisa]

It can't become a dev-dependency unfortunately because integration of loom requires changes to the library code. It could possibly become an optional dependency, though I do not currently have capacity to work on this in the near future.