Add linter that detects sequential independent awaits

[fredemmott]

e.g:

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

should suggest autofix:

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

... but this is fine:

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

[lexidor]

Update, use concurrent{} instead of the special function va(). va() is no longer supported.