Remove extern crate

Now we have edition 2018 we do not need to use `macro_use` or `extern
crate`; `pub use` works with macros. Notable exceptions are `alloc` and
`test`. Also leave the serde rename because touching it opens a can of
worms.

Author: tcharding

src/lib.rs

@@ -59,32 +59,18 @@ extern crate test;
 #[cfg(feature = "no-std")]
 #[macro_use]
 extern crate alloc;
-#[cfg(feature = "no-std")]
-extern crate core2;
 
-// Re-exported dependencies.
+// Re-export dependencies we control.
 pub use bitcoin_hashes as hashes;
-pub extern crate bech32;
-pub extern crate secp256k1;
-
-#[cfg(feature = "no-std")]
-extern crate hashbrown;
+pub use secp256k1;
+pub use bech32;
+#[cfg(feature="bitcoinconsensus")]
+pub use bitcoinconsensus;
 
-#[cfg(feature = "base64")]
-#[cfg_attr(docsrs, doc(cfg(feature = "base64")))]
-pub extern crate base64;
-
-#[cfg(feature = "bitcoinconsensus")]
-extern crate bitcoinconsensus;
 #[cfg(feature = "serde")]
 #[macro_use]
 extern crate actual_serde as serde;
-#[cfg(all(test, feature = "serde"))]
-extern crate bincode;
-#[cfg(all(test, feature = "serde"))]
-extern crate serde_json;
-#[cfg(all(test, feature = "serde"))]
-extern crate serde_test;
+
 
 #[cfg(test)]
 #[macro_use]