Improve crate root re-exports

When we moved to edition 2018 the use of `extern` became unnecessary and
we moved to using `pub use` for re-exports. It was observed however that
`pub extern crate` is more readable.

Improve crate root re-exports by doing:

- Use `pub extern crate foo` to re-export foo.
- Fix docs attribute for optional dependency `bitcoinconsensus`.
- Re-order to how rustfmt would put them.

bitcoin/src/lib.rs

@@ -63,20 +63,18 @@ extern crate test;
 #[macro_use]
 extern crate alloc;
 
-// Re-export dependencies we control.
-#[cfg(feature = "bitcoinconsensus")]
-pub use bitcoinconsensus;
-pub use {bech32, bitcoin_hashes as hashes, secp256k1};
-
-// Re-export base64 when enabled
 #[cfg(feature = "base64")]
 #[cfg_attr(docsrs, doc(cfg(feature = "base64")))]
-pub use base64;
+pub extern crate base64;
-
+pub extern crate bech32;
-// Re-export hashbrown when enabled
+pub extern crate bitcoin_hashes as hashes;
+#[cfg(feature = "bitcoinconsensus")]
+#[cfg_attr(docsrs, doc(cfg(feature = "bitcoinconsensus")))]
+pub extern crate bitcoinconsensus;
 #[cfg(feature = "hashbrown")]
 #[cfg_attr(docsrs, doc(cfg(feature = "hashbrown")))]
-pub use hashbrown;
+pub extern crate hashbrown;
+pub extern crate secp256k1;
 
 #[cfg(feature = "serde")]
 #[macro_use]