Fix conflicting async io feature compilation

[filipeom]

Previously, enabling with-tokio would also activate the default feature (with-futures), because Cargo includes default features by design. This would lead to a compilation error due to duplicate definitions from both use statements. Example:

$ cargo build --features with-tokio
error[E0252]: the name `AsyncRead` is defined multiple times
 --> src/ascii.rs:8:34
  |
5 | use futures::io::{AsyncBufReadExt, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt, BufReader};
  |                                    --------- previous import of the trait `AsyncRead` here
...
8 | use tokio::io::{AsyncBufReadExt, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt, BufReader};
  |                                  ^^^^^^^^^ `AsyncRead` reimported here
  |
  = note: `AsyncRead` must be defined only once in the type namespace of this module
help: you can use `as` to change the binding name of the import
  |
8 | use tokio::io::{AsyncBufReadExt, AsyncRead as OtherAsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt, BufReader};
  |                                            +++++++++++++++++

error[E0252]: the name `AsyncReadExt` is defined multiple times
 --> src/ascii.rs:8:45
  |
5 | use futures::io::{AsyncBufReadExt, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt, BufReader};
  |                                               ------------ previous import of the trait `AsyncReadExt` here
...
8 | use tokio::io::{AsyncBufReadExt, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt, BufReader};
  |                                             ^^^^^^^^^^^^ `AsyncReadExt` reimported here
  |
  = note: `AsyncReadExt` must be defined only once in the type namespace of this module
help: you can use `as` to change the binding name of the import
  |
8 | use tokio::io::{AsyncBufReadExt, AsyncRead, AsyncReadExt as OtherAsyncReadExt, AsyncWrite, AsyncWriteExt, BufReader};
  |                                                          ++++++++++++++++++++

error[E0252]: the name `AsyncWrite` is defined multiple times
 --> src/ascii.rs:8:59
  |
5 | use futures::io::{AsyncBufReadExt, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt, BufReader};
  |                                                             ---------- previous import of the trait `AsyncWrite` here
...
8 | use tokio::io::{AsyncBufReadExt, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt, BufReader};
  |                                                           ^^^^^^^^^^ `AsyncWrite` reimported here
  |
  = note: `AsyncWrite` must be defined only once in the type namespace of this module
help: you can use `as` to change the binding name of the import
  |
8 | use tokio::io::{AsyncBufReadExt, AsyncRead, AsyncReadExt, AsyncWrite as OtherAsyncWrite, AsyncWriteExt, BufReader};
  |                                                                      ++++++++++++++++++

error[E0252]: the name `AsyncWriteExt` is defined multiple times
 --> src/ascii.rs:8:71
  |
5 | use futures::io::{AsyncBufReadExt, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt, BufReader};
  |                                                                         ------------- previous import of the trait `AsyncWriteExt` here
...
8 | use tokio::io::{AsyncBufReadExt, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt, BufReader};
  |                                                                       ^^^^^^^^^^^^^ `AsyncWriteExt` reimported here
  |
  = note: `AsyncWriteExt` must be defined only once in the type namespace of this module
help: you can use `as` to change the binding name of the import
  |
8 | use tokio::io::{AsyncBufReadExt, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt as OtherAsyncWriteExt, BufReader};
  |                                                                                     +++++++++++++++++++++

error[E0252]: the name `BufReader` is defined multiple times
 --> src/ascii.rs:8:86
  |
5 | use futures::io::{AsyncBufReadExt, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt, BufReader};
  |                                                                                        --------- previous import of the type `BufReader` here
...
8 | use tokio::io::{AsyncBufReadExt, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt, BufReader};
  |                                                                                      ^^^^^^^^^ `BufReader` reimported here
  |
  = note: `BufReader` must be defined only once in the type namespace of this module
help: you can use `as` to change the binding name of the import
  |
8 | use tokio::io::{AsyncBufReadExt, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt, BufReader as OtherBufReader};
  |                                                                                                +++++++++++++++++

warning: unused imports: `AsyncBufReadExt`, `AsyncReadExt`, `AsyncRead`, `AsyncWriteExt`, `AsyncWrite`, and `BufReader`
 --> src/ascii.rs:8:17
  |
8 | use tokio::io::{AsyncBufReadExt, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt, BufReader};
  |                 ^^^^^^^^^^^^^^^  ^^^^^^^^^  ^^^^^^^^^^^^  ^^^^^^^^^^  ^^^^^^^^^^^^^  ^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

For more information about this error, try `rustc --explain E0252`.
warning: `memcache-async` (lib) generated 1 warning
error: could not compile `memcache-async` (lib) due to 6 previous errors; 1 warning emitted

This commit replaces the separate #[cfg] blocks with a single cfg_if macro. This creates a prioritized selection for the io backend.