Check for sequential independent awaits, replace with `\HH\Asio\va`

[fredemmott]

e.g:

$a = await foo();
$b = await bar();

should be:

list($a, $b) = await \HH\Asio\va(foo(), bar());

... but this is fine:

$a = await foo();
$b = await bar($a->getFoo());

Related: hhvm/hhast#8

[justinwray]

+1

We actually went through the code and did this back in #325, but I am sure some more have cropped up since then.

[fredemmott]

Filed #579 and hhvm/hhast#8; given the restrictions on when await is valid, should be able to get a good enough idea of when variables are independent to detect this automatically