Switch library to async

[SplittyDev]

This PR switches the whole library over to async.

It introduces three new features:

• blocking for stdlib I/O
• tokio1 for full tokio support
• async-std1 for full async-std support

The blocking feature is essentially a bring your own runtime feature, where standard Rust stdlib I/O functions are used. The behavior will essentially be the same as in the old non-async version of the library, but it still needs an async runtime to run. However, that way it can be used with non-supported async runtimes such as smol, and theoretically it should also work with the futures executor since it doesn't need a reactor.

Architectural Concerns

Right now, we're not enabling any feature by default, so when just adding it without any explicitly enabled features, the crate won't compile. I'm not sure that's the best solution, but defaulting to any runtime would require also passing default-features = false in addition to the actual feature, so that doesn't sound great either.

Update: I think I've found a great way to deal with this. The default feature now enables an implicit-blocking feature, which acts exactly like the explicit blocking feature, except when either tokio1 or async-std1 is present, in which case it acts like it doesn't exist.

The code for this is a bit funky, since we have to deal with constructs such as the following:

#[cfg(any(
  feature = "blocking",
  all(
    feature = "implicit-blocking",
    not(any(feature = "tokio1", feature = "async-std1")),
  )
))] {
  // Business as usual
}

But that shouldn't really matter. It's not too hard to maintain, and it unlocks a great way of dealing with this.
So, everything is blocking by default now, unless an explicit runtime feature is enabled.

[Neotamandua]

Change feature naming to:
rt_tokio_1
rt_async-std_1