Use a single iterator to construct a BlindedPath

Use a single iterator to construct a BlindedPath

Instead of using separate iterators for pubkeys and TLVs, use an
iterator that yields a tuple of them. This allows for holding a mutable
reference to the TLVs such that they can be padded. With two iterators,
the pubkey iterator would have a reference to the ForwardNode slice when
constructing a payment path. However, this would prevent holding mutable
references in the TLVs iterator.