deferred subroutine syntax for futures

 Future::AsyncAwait provides syntax for deferring and resuming subroutines
 while waiting for Futures to complete. This syntax aims to make code that
 performs asynchronous operations using futures look neater and more
 expressive than simply using then chaining and other techniques on the
 futures themselves. It is also a similar syntax used by a number of other
 languages; notably C# 5, EcmaScript 6, Python 3, and lately even Rust is
 considering adding it.
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 The new syntax takes the form of two new keywords, async and await.
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 STABILITY WARNING: This module is still relatively new and under active
 development. While it now seems relatively stable enough for most use-cases,
 there may still be a number of memory leaks left in it, especially if
 still-pending futures are abandoned.