Identify more cases of useless `into_iter()` calls

If the type of the result of a call to `IntoIterator::into_iter()`
and the type of the receiver are the same, then the receiver
implements `Iterator` and `into_iter()` is the identity function.

The call to `into_iter()` may be removed in all but two cases:

- If the receiver implements `Copy`, `into_iter()` will produce
  a copy of the receiver and cannot be removed. For example,
  `x.into_iter().next()` will not advance `x` while `x.next()` will.
- If the receiver is an immutable local variable and the call to
  `into_iter()` appears in a larger expression, removing the call to
  `into_iter()` might cause mutability issues. For example, if `x`
  is an immutable local variable, `x.into_iter().next()` will
  compile while `x.next()` will not as `next()` receives
  `&mut self`.

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