Switch back to futures from crates.io (#113)

Doing so requires copying the `current_thread` executor from GitHub into
the repo.

src/executor/mod.rs

@@ -0,0 +1,8 @@
+//! Task execution utilities.
+//!
+//! This module only contains `current_thread`, an executor for multiplexing
+//! many tasks on a single thread.
+
+pub mod current_thread;
+mod scheduler;
+mod sleep;