Executor API V2.

- It's no longer possible to call run() reentrantly from within a task (soundness issue)
- it's now possible to spawn Send tasks across threads (SendSpawner, #37)

.vscode/settings.json

@@ -1,4 +1,5 @@
 {
+  "rust-analyzer.assist.importMergeBehavior": "last",
   "editor.formatOnSave": true,
   "rust-analyzer.cargo.allFeatures": false,
   "rust-analyzer.checkOnSave.allFeatures": false,
@@ -8,4 +9,4 @@
     "**/.git/subtree-cache/**": true,
     "**/target/**": true
   }
-}
+}