descriptor: support elements confidential wallet policies

[jgriffiths]

    These are the same as existing wallet policies except that the blinding
    key placeholder must be '@B`, which must be given first in the policy
    key map when parsing.
    
    This allows the blinding scheme and key to be presented separately from
    the rest of the policy which will then be identical (modulo el-prefixes)
    between elements and bitcoin for the same wallet structure.
    
    This also allows the distinction between blinding keys and policy keys
    to remain intact. Blinding keys are qualitatively different from
    derivation keys; they may for example be a real pubkey, raw entropy,
    and/or/in addition to an algorithm that operates on the result of the
    policy for given derivation indices (in the case of deterministic
    blinding). For these reasons blinding keys remain outside standard key
    iteration in a parsed descriptor.