Introduce ext_println to contract runtime (#2239)

* Implement `ext_println` in contract runtime

* Only allow contracts to import `ext_println` on dev chains

* Configure dev chain to allow contracts with `ext_println`

* Increment spec version

* Docs

* Rename config to the more specific enable_println

substrate/node/runtime/src/lib.rs

@@ -59,7 +59,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	spec_name: create_runtime_str!("node"),
 	impl_name: create_runtime_str!("substrate-node"),
 	authoring_version: 10,
-	spec_version: 58,
+	spec_version: 59,
 	impl_version: 59,
 	apis: RUNTIME_API_VERSIONS,
 };