Move `impl_std_error` to `bitcoin-internals`

The macro is useful for all other crates thus it is moved to the
internals crate in this commit.

bitcoin/src/error.rs

@@ -2,22 +2,6 @@
 
 //! Contains error types and other error handling tools.
 
-pub use crate::parse::ParseIntError;
+pub(crate) use internals::impl_std_error;
 
-/// Impls std::error::Error for the specified type with appropriate attributes, possibly returning
+pub use crate::parse::ParseIntError;
-/// source.
-macro_rules! impl_std_error {
-    // No source available
-    ($type:ty) => {
-        #[cfg(feature = "std")]
-        impl std::error::Error for $type {}
-    };
-    // Struct with $field as source
-    ($type:ty, $field:ident) => {
-        #[cfg(feature = "std")]
-        impl std::error::Error for $type {
-            fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { Some(&self.$field) }
-        }
-    };
-}
-pub(crate) use impl_std_error;

internals/src/error.rs

@@ -26,3 +26,21 @@ macro_rules! write_err {
         }
     }
 }
+
+/// Impls std::error::Error for the specified type with appropriate attributes, possibly returning
+/// source.
+#[macro_export]
+macro_rules! impl_std_error {
+    // No source available
+    ($type:ty) => {
+        #[cfg(feature = "std")]
+        impl std::error::Error for $type {}
+    };
+    // Struct with $field as source
+    ($type:ty, $field:ident) => {
+        #[cfg(feature = "std")]
+        impl std::error::Error for $type {
+            fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { Some(&self.$field) }
+        }
+    };
+}