rename Read to ReadOnce and expose it

The other read futures (read_exact, read_until, etc.) all expose their
concrete future types so that function signatures can return them, but
until now `read()` didn't. Exposing it with the name "Read" causes
naming conflicts with the std::io::Read trait, so the easiest thing to
do is to just change the name. Importing std::io::Read under a different
name would've been an option too, but that would probably be annoying
for consumers in the same way it's annoying for us.

The original PR (https://github.com/tokio-rs/tokio-core/pull/29) decided
that "read" was a better name than "read_some", so I'm leaving the top
level functions unchanged. I don't have a strong opinion about it one
way or the other, but I *do* think it's worth bikeshedding a little bit.
Python's asyncio library actually ended up with a very similar issue
around naming inconsistency between the sync and async worlds, and we
can hopefully avoid repeating that: https://bugs.python.org/issue22279

src/io/mod.rs

@@ -44,7 +44,7 @@ pub use self::copy::{copy, Copy};
 pub use self::flush::{flush, Flush};
 pub use self::read_exact::{read_exact, ReadExact};
 pub use self::read_to_end::{read_to_end, ReadToEnd};
-pub use self::read::read;
+pub use self::read::{read, ReadOnce};
 pub use self::read_until::{read_until, ReadUntil};
 pub use self::split::{ReadHalf, WriteHalf};
 pub use self::window::Window;