Application improvements

[Max-Meldrum]

This PR contains a bunch of improvements regarding the creation of an Arcon application.

Removes:

• AssembledApplication
• arcon attr requirement on #[derive(Arcon)]
• unsafe_flight feature

Introduces:

• ApplicationBuilder
• Extension traits ToStreamExt, ToSinkExt, and ToBuilderExt
• #[tokio::main] inspired app macro
• naivé sink type

You no longer create streams from an Application struct. Users can convert types that implement ToStreamExt into a Stream<T> type and then later turn it into a sink type.

Using app macro

#[arcon::app(name = "example")]
fn main() {
    (0..10u64)
        .to_stream(|conf| conf.set_arcon_time(ArconTime::Process))
        .filter(|x| *x > 50)
        .map(|x| x * 10)
        .print()
}

Without app macro

let mut builder: ApplicationBuilder = (0..10u64)
     .to_stream(|conf| conf.set_arcon_time(ArconTime::Process))
     .filter(|x| *x > 50)
     .map(|x| x * 10)
     .print()
     .builder();

let mut app: Application = builder
   .name("example")
   .build();

app.run_and_block();