hazmat feature and module for exporting crate internals

[prestwich]

Some things like BatchFuture, RouteFuture and much of the Router interface are public to allow testing and implementing custom transports. These should be hidden behind some sort of feature, as rust-analyzer will currently suggest importing them, which is very rarely what users actually want

#[cfg_attr(not(feature = "hazmat"), doc(hidden))]
pub mod hazmat {
    pub use ....;
}