Add StreamExt::map_while

[nanoqsh]

The proposal to add a similar method from std map_while for stream.
This would be convenient to reduce this:

stream.map(|x| u32::checked_div(16, x))
    .take_while(|x| future::ready(x.is_some()))
    .map(|x| x.unwrap())

to this:

stream.map_while(|x| future::ready(u32::checked_div(16, x)))