Tr(): rawnode() and rawleaf() support

Could be useful for inheritance/escrow type services: “this isn’t my money, so I don’t care about other spend paths, but in an emergency, the spend path I do know about can be used to help that person recover their money even if they’ve lost all their keys”.

Could be useful for places where you’re using multiple wallets for some funds – maybe your main wallet understands some super clever contracts that you want to put in a tapscript, but your backup (hardware? cold?) wallet doesn’t and can only spend via the key path; in that case all the backup wallet needs to know is the ipk and a rawnode leaf.