Enhancements for `manual_flatten`

[camsteffen]

Enhancements for manual_flatten

1. for_each

   Before:

   iter.for_each(|o| {
       if let Some(x) = o { //.. }
   })

   After

   iter.flatten().for_each(|o| //..)

2. nested loops

   Before:

   for x in vec![vec![1]] {
       for y in x {
           //..  x is not used again
       }
   }

   After:

   for x in vec![vec![1]].into_iter().flatten() {
       // ..
   }

[chrisranderson]

@rustbot claim

[maxclaus]

@rustbot claim

[ayamdobhal]

@rustbot claim

[ayamdobhal]

Before I work on this. Has this been resolved by rust-lang/rust#82687 or do we still need to work on this?