Add example usage of new _global API

src/key.rs

@@ -466,6 +466,20 @@ impl PublicKey {
         }
     }
 
+    /// This is just a demo of the new _global API
+    pub fn _from_secret_key(sk: &SecretKey) -> PublicKey {
+        unsafe {
+            crate::context::_global::with_global_signing_context(|ctx| {
+                let mut pk = ffi::PublicKey::new();
+                // We can assume the return value because it's not possible to construct
+                // an invalid `SecretKey` without transmute trickery or something.
+                let res = ffi::secp256k1_ec_pubkey_create(ctx, &mut pk, sk.as_c_ptr());
+                debug_assert_eq!(res, 1);
+                PublicKey(pk)
+            })
+        }
+    }
+
     /// Creates a new public key from a [`SecretKey`] and the global [`SECP256K1`] context.
     #[inline]
     #[cfg(feature = "global-context")]