rt: simplify rt-* features (#2949)

tokio:

    merge rt-core and rt-util as rt
    rename rt-threaded to rt-multi-thread

tokio-util:

    rename rt-core to rt

Closes #2942

tests-integration/Cargo.toml

@@ -8,16 +8,16 @@ publish = false
 [features]
 full = [
     "macros",
-    "rt-core",
-    "rt-threaded",
+    "rt",
+    "rt-multi-thread",
 
     "tokio/full",
     "tokio-test"
 ]
 macros = ["tokio/macros"]
 sync = ["tokio/sync"]
-rt-core = ["tokio/rt-core"]
-rt-threaded = ["rt-core", "tokio/rt-threaded"]
+rt = ["tokio/rt"]
+rt-multi-thread = ["rt", "tokio/rt-multi-thread"]
 
 [dependencies]
 tokio = { path = "../tokio" }

tests-integration/tests/macros_main.rs

@@ -1,4 +1,4 @@
-#![cfg(all(feature = "macros", feature = "rt-core"))]
+#![cfg(all(feature = "macros", feature = "rt"))]
 
 #[tokio::main]
 async fn basic_main() -> usize {