Rename idiom lints to migration lints

Author: Manishearth

src/librustc_lint/lib.rs

@@ -178,7 +178,7 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
                     UNUSED_PARENS);
 
     add_lint_group!(sess,
-                    "rust_2018_idioms",
+                    "rust_2018_migration",
                     BARE_TRAIT_OBJECT,
                     UNREACHABLE_PUB);