Put loom behind cfg(oneshot_loom) again

[faern]

Followup to #43. Loom is still optional, taking it out of the dependency graph and not polluting Cargo.lock for downstream users. But also "hide" the feature a bit more. It's still exposed in cargo metadata sadly. And building with cargo build --all-features will enable the loom feature. However just enabling the feature does nothing.

This puts the dependency also behind [target.'cfg(oneshot_loom)'.dependencies] and all the code that is conditional on relying on loom vs std now use cfg(oneshot_loom) instead of cfg(feature = "loom"). This (again) makes it way harder for downstream users to accidentally compile oneshot with loom integration turned on.

So we are back to how things were before #43 but with the following differences:

• Due to being optional, loom is taken out of the default dependency tree
• The cfg flag is renamed from loom to oneshot_loom to not collide with other loom-enabled crates, since RUSTFLAGS are global.

Ping @Wuelle :)