Fix buggy cfg in rustdocs

In b9643bf3e9 we introduced an incorrect
`cfg` attribute, that has just shown up, no clue why clippy only just
presented me with this error now. Anywho, the current code is buggy and
the rustdoc tests are never being run.

Fix `cfg` attribute to use the feature name correctly and fix the
imports so the code runs.

Maintain the explicit `main` so that we can return an error using the
`?` operator. Remove the empty `main` because its not needed anymore,
it is a hang-over from Rust back in the day (before main was
automatically added, IIUC).

hashes/src/lib.rs

@@ -11,7 +11,7 @@
 //!
 //! Hashing a single byte slice or a string:
 //!
-//! ```rust
+//! ```
 //! use bitcoin_hashes::Sha256;
 //!
 //! let bytes = [0u8; 5];
@@ -22,39 +22,32 @@
 //!
 //! Hashing content from a reader:
 //!
-//! ```rust
-//! #[cfg(std)]
-//! # fn main() -> std::io::Result<()> {
+//! ```
+//! #[cfg(feature = "std")] {
+//! use bitcoin_hashes::Sha256;
 //! let mut reader: &[u8] = b"hello"; // in real code, this could be a `File` or `TcpStream`
 //! let mut engine = Sha256::engine();
-//! std::io::copy(&mut reader, &mut engine)?;
-//! let hash = Sha256::from_engine(engine);
-//! # Ok(())
+//! std::io::copy(&mut reader, &mut engine).unwrap();
+//! let _hash = Sha256::from_engine(engine);
 //! # }
-//!
-//! #[cfg(not(std))]
-//! # fn main() {}
 //! ```
 //!
 //!
 //! Hashing content by [`std::io::Write`] on `HashEngine`:
 //!
-//! ```rust
-//! #[cfg(std)]
-//! # fn main() -> std::io::Result<()> {
-//! let mut part1: &[u8] = b"hello";
-//! let mut part2: &[u8] = b" ";
-//! let mut part3: &[u8] = b"world";
+//! ```
+//! #[cfg(feature = "std")] {
+//! use std::io::Write as _; // Or `bitcoin-io::Write` if `bitcoin-io` feature is enabled.
+//! use bitcoin_hashes::Sha256;
+//! let part1: &[u8] = b"hello";
+//! let part2: &[u8] = b" ";
+//! let part3: &[u8] = b"world";
 //! let mut engine = Sha256::engine();
-//! engine.write_all(part1)?;
-//! engine.write_all(part2)?;
-//! engine.write_all(part3)?;
-//! let hash = Sha256::from_engine(engine);
-//! # Ok(())
+//! engine.write_all(part1).expect("engine writes don't error");
+//! engine.write_all(part2).unwrap();
+//! engine.write_all(part3).unwrap();
+//! let _hash = Sha256::from_engine(engine);
 //! # }
-//!
-//! #[cfg(not(std))]
-//! # fn main() {}
 //! ```
 
 #![cfg_attr(not(feature = "std"), no_std)]