Make futures default and tokio optional

[gsserge]

This PR implements the suggestion in #9 (comment)

Couldn't get the doctest to work with Tokio being optional, so just ignored it. Happy to take any suggestions how to actually make it work.

@geigerzaehler let me know what you think, thanks!

[seanpianka]

It seems like you're able to conditionally include documentation using something like the following:

#![cfg_attr(feature = "alpha", doc = "
# Examples
```rust
fn alpha() {}
\``` # ignore leading slash
")]

[geigerzaehler]

lgtm. Just one comment

[geigerzaehler]

Instead of ignoring this example you can use the futures traits

diff --git a/src/lib.rs b/src/lib.rs
index 8593b93..345f539 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -7,3 +7,3 @@
 //! use async_pipe;
-//! use tokio::io::{AsyncWriteExt, AsyncReadExt};
+//! use futures::io::{AsyncWriteExt, AsyncReadExt};
 //!

Then the example compiles with the default flags.

[gsserge]

Cool, thanks!

[gsserge]

Interesting, the futures traits do not play well with tokio which is still used in the doc test for spawning and executing. On macOS things are fine though, but on Linux the doc test never finishes (https://github.com/routerify/async-pipe-rs/actions/runs/1093123889).

I've tried to play with cfg attrs and conditional compilation, but unsuccessfully, and honestly it's not really worth the trouble. I'm just gonna revert back to tokio and because our CI uses cargo test --all-features it's actually fine.