[Rust] Add a prelude

[pschafhalter]

To use ERDOS for a simple dataflow, users currently need to do the following:

use erdos::{
    self,
    dataflow::{
        message::*,
        operators::{JoinOperator, MapOperator},
        stream::{ExtractStream, IngestStream, WriteStreamT},
        Operator, OperatorConfig, ReadStream, WriteStream,
    },
};

This should be simplified to use erdos::prelude::* to cover the essential imports, like in other libraries.