Proposal: "yield all elements" statement

[masonwheeler]

This isn't incredibly common, but it pops up often enough when working with enumerables, that it would be nice to have a simpler way to write it:

foreach (var foo in bar)
{
    yield return foo;
}

Would it be possible to get a way to represent this pattern as a single statement that would lower to the loop above? Something like yield in bar; or yield from bar;, for example.

[333fred]

Dupe of #378.