Since the iterator created by `IntoIterator` should be called `IntoIter`
we move the whole `TaprootMerkleBranch` to its own module which contains
the type to avoid confusion. This has an additional benefit of reducing
the scope where the invariant could be broken. This already uncovered
that our internal code was abusing access to the private field (although
the code was correct).
To implement the iterator we simply delegate to `vec::IntoIter`,
including overriding the default method which are likely to be
implemented by `Vec` more optimally. We avoid exposing `vec::IntoIter`
directly since we may want to change the representation (e.g. to
`ArrayVec`).
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