Expose .iter_mut() from SelectAll

[yshui]

Currently there is no way to modify the set of streams in SelectAll, yet the underlying FuturesUnordered does have this capability.

I wonder if it's possible to expose the same thing in SelectAll as well?

[ibraheemdev]

when #2423 is merged IntoIter and IterPinRef should be exposed as well, so this issue should be kept open until then