rename Error::iter_chain() and remove Error::iter_sources()

Rename
* Error::iter_chain() -> Error::chain()
* ErrorIter -> Chain

Removed
* Error::iter_sources()

according to
https://github.com/rust-lang/rust/issues/58520

Rationale:

   1. Such iterators are helpful. They should better be stabilized sooner
      than later.
   2. self should be included. It is easy to .skip(1) it.
      Not including self is harmful because it is harder to add self
      to the iterator than to remove it.
   3. The chosen name should be telling and reflect the fact that self is
      included. `.chain()` was chosen because the iterator iterates over
      the chain of errors that is somehow included in self.
   4. The resulting iterator is named `Chain` because the `error::Chain`
      is what we want to have.

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