Add async feature and rework Dispatcher

[yukibtc]

No description provided.

[JoeJoeTV]

This looks good to me. I like the changes to separate the "method" out a bit more.

Things that would be nice to add:

• Since the client/agent is now passed to the Dispatcher, maybe one could add some method to pass custom settings to it? For me, this would be useful to set the browser agent string of the client, since it is currently empty, at least with ureq. Maybe a function that is given a closure that is given the client/agent builder and can call additional methods on it, before the rest is done in the build_* function? Or by optionally passing an existing builder to the build_* function.
• A builder function in the dispatcher module itself, which just returns DispatcherBuilder::new(). I have seen this in other libraries and I like it, even though it's just a small thing. Something like the following maybe:

  pub fn builder<S>(url: S) -> DispatcherBuilder
  where
      S: Into<String>,
  {
      DispatcherBuilder::new::<S>(url)
  }

[yukibtc]

Since the client/agent is now passed to the Dispatcher, maybe one could add some method to pass custom settings to it? For me, this would be useful to set the browser agent string of the client, since it is currently empty, at least with ureq. Maybe a function that is given a closure that is given the client/agent builder and can call additional methods on it, before the rest is done in the build_* method?

Yes, good idea. This can be done in another PR

A builder function in the dispatcher module itself, which just returns DispatcherBuilder::new(). I have seen this in other libraries and I like it, even though it's just a small thing.

Was already available the Dispatcher::builder, which return the DistaptcherBuilder. I like it too, but in this PR I deprecated it because require to specify a generic when calling Dispatcher::builder (i.e., Dispatcher::<Async>::builder). But there is a "trick" to allow to continue to use it without breaking changes:

#[derive(Debug, Clone)]
pub struct Dispatcher<T = ()> // <-- set a default value for the generic
where
    T: Clone,
{
    url: Url,
    inner: T,
}

// Impl the `builder` method for the `Dispatcher<()>`, allowing to call `Dispatcher::builder`.
impl Dispatcher {
    pub fn builder<S>(url: S) -> DispatcherBuilder
    where
        S: Into<String>,
    {
        DispatcherBuilder::new(url)
    }
}

[JoeJoeTV]

Yes, good idea. This can be done in another PR

I guess first, this pull-request should be completed before that, then I could create that.

A builder function in the dispatcher module itself, which just returns DispatcherBuilder::new(). I have seen this in other libraries and I like it, even though it's just a small thing.

Was already available the Dispatcher::builder, which return the DistaptcherBuilder. I like it too, but in this PR I deprecated it because require to specify a generic when calling Dispatcher::builder (i.e., Dispatcher::<Async>::builder). But there is a "trick" to allow to continue to use it without breaking changes:

#[derive(Debug, Clone)]
pub struct Dispatcher<T = ()> // <-- set a default value for the generic
where
    T: Clone,
{
    url: Url,
    inner: T,
}

// Impl the `builder` method for the `Dispatcher<()>`, allowing to call `Dispatcher::builder`.
impl Dispatcher {
    pub fn builder<S>(url: S) -> DispatcherBuilder
    where
        S: Into<String>,
    {
        DispatcherBuilder::new(url)
    }
}

I know that there was a function inside the struct implementation before, but what I'm talking about is just putthing the method in the dispatcher module itself and not in the struct. From what I see, that also doesn't require the trick. An example would be ureq in the current stable version: https://github.com/algesten/ureq/blob/ba4555d54fd72e16fc737f7e8fcc812efb131260/src/lib.rs#L492

[yukibtc]

I know that there was a function inside the struct implementation before, but what I'm talking about is just putthing the method in the dispatcher module itself and not in the struct. From what I see, that also doesn't require the trick. An example would be ureq in the current stable version: https://github.com/algesten/ureq/blob/ba4555d54fd72e16fc737f7e8fcc812efb131260/src/lib.rs#L492

Ahh, sure, I'll add this!