This change allows adding a length prefix to a serialized TLV. It will
be particularly useful for serializing all 'scb_chan' entries in
the 'emergency.recover' file.
Key Changes:
- Removed the need to loop in towire_tlv and fromwire_tlv, so the if conditions have been modified accordingly.
- During serialization, the length of the TLV is calculated before appending it, and it is stored in a temporary variable.
- For fromwire_tlv, only a simple length adjustment is required, and no loop is needed here either.
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