Merge rust-bitcoin/rust-bitcoin#3838: primitives: Add core re-exports

79791e7444 primitives: Add core re-exports (Tobin C. Harding)

Pull request description:

  This is not strictly needed but we would like to copy the public macros from `units` to primitives and they use `$crate::_export` paths.

ACKs for top commit:
  apoelstra:
    ACK 79791e7444326d284bdf42c40926f6b05a6b65f7; successfully ran local tests; sure, lgtm

Tree-SHA512: cdf683041dca07bc53ae8e70806cfb74ef79e94b8699112774e3bfb3efd29cc270bd24ab03a448197edd1d143dec07d18d9a3a92d74097d87478d40954392ee4

primitives/src/lib.rs

@@ -33,6 +33,14 @@ extern crate std;
 #[macro_use]
 extern crate serde;
 
+#[doc(hidden)]
+pub mod _export {
+    /// A re-export of `core::*`.
+    pub mod _core {
+        pub use core::*;
+    }
+}
+
 pub mod block;
 pub mod locktime;
 pub mod merkle_tree;