executor: move current-thread into crate (#1447)

The `CurrentThread` executor is exposed using a feature flag.

Refs: #1264

azure-pipelines.yml

@@ -66,8 +66,8 @@ jobs:
     crates:
       tokio-buf: []
       tokio-codec: []
-      tokio-current-thread: []
-      tokio-executor: []
+      tokio-executor:
+        - current-thread
       tokio-io:
         - util
       tokio-sync:
@@ -78,6 +78,16 @@ jobs:
         - async-traits
       tokio-test: []
 
+# Test compilation failure
+- template: ci/azure-test-stable.yml
+  parameters:
+    name: test_features
+    displayName: Test feature flags
+    rust: $(nightly)
+    crates:
+      ui-tests:
+        - executor-without-current-thread
+
 # - template: ci/azure-cargo-check.yml
 #   parameters:
 #     name: features