Remove all default features. Making them opt-in

[faern]

The reason for this is outlined in the comment in Cargo.toml in the PR content itself.

Before this can be merged, I need to add fancy documentation helper tags that makes everything show up neatly on docs.rs.

We can also consider dropping the async feature and always support at least async receiving. It will compile in some unnecessary complexity, but maybe it's negligible compared to the complexity of keeping some of the code conditionally compiled etc. We should probably not remove the std feature, since it's needed to allow making the crate no_std. However, it can be discussed whether it's better renamed to blocking or similar. Since what it allows is blocking receive operations, which happens to have the side effect of requiring the standard library.