Runtime logging. (#3821)

* Implement Printable for tuples.

* Add debugging function.

* Add debug 1.

* Implement  for everything.

* RuntimeDebug derive.

* Introduce RuntimeDebug.

* Add some dummy logging.

* Replace RuntimeDebug with Debug.

* Revert "Replace RuntimeDebug with Debug."

This reverts commit bc47070a8cb30241b2b590b2fa29fd195088162f.

* Working on Debug for all.

* Fix bounds.

* Add debug utils.

* Implement runtime logging.

* Add some docs and clean up.

* Clean up derives.

* Fix custom derive impl.

* Bump runtime.

* Fix long lines.

* Fix doc test.

* Use CARGO_CFG_STD.

* Revert "Use CARGO_CFG_STD."

This reverts commit ea429566de18ed0fa052571b359eb9826a64a9f4.

* Use parse_macro_input

* Update lockfile.

* Apply review suggestions.

* Remove stray re-export.

* Add no-std impl.

* Update lockfile.

substrate/core/sr-io/src/lib.rs

@@ -33,6 +33,7 @@ use primitives::{
 	offchain::{
 		Timestamp, HttpRequestId, HttpRequestStatus, HttpError, StorageKind, OpaqueNetworkState,
 	},
+	LogLevel,
 };
 
 /// Error verifying ECDSA signature
@@ -158,6 +159,20 @@ export_api! {
 		fn print_utf8(utf8: &[u8]);
 		/// Print any `u8` slice as hex.
 		fn print_hex(data: &[u8]);
+
+		/// Request to print a log message (stderr) on the host.
+		///
+		/// Note that this will be only displayed if the host
+		/// is enabed to display log messages with given
+		/// level and target.
+		///
+		/// Instead of using directly, prefer setting up `RuntimeLogger`
+		/// and using `log` macros.
+		fn log(
+			level: LogLevel,
+			target: &[u8],
+			message: &[u8]
+		);
 	}
 }