Expose AsyncTerminator publicly

[lberezy]

Resolves #122

[wolf4ood]

@lberezy sorry for the delay

Thanks :)

[criminosis]

@wolf4ood how does one invoke a use statement to get AsyncTerminator?

Doing this use gremlin_client::aio::process::traversal::remote::AsyncTerminator yields:

error[E0603]: module `process` is private
 --> src/lib.rs:8:26
  |
8 | use gremlin_client::aio::process::traversal::remote::AsyncTerminator;
  |                          ^^^^^^^ private module

It almost feels like the aio module's mod file should have something like pub use process::traversal::AsyncTerminator; in this PR? Still getting the hang of Rust though, not sure if that would then conflict with pub(crate) mod process;

Ultimately I'm trying to do something like this:

fn get_traversal(&self) ->  GraphTraversalSource<AsyncTerminator>{
        traversal().with_remote_async(self.client.clone())
    }

That is a method on a struct I've written that is wrapping a GremlinClient to handle some common execution query logic.