Failure to parse `pk(xprv.../*)` descriptor

[brunoerg]

See bitcoinfuzz/bitcoinfuzz#70.

The following descriptor pk(xprv9s21ZrQH143K31xYSDQpPDxsXRTUcvj2iNHm5NUtrGiGG5e2DtALGdso3pGz6ssrdK4PFmM8NSpSBHNqPqm55Qn3LqFtT2emdEXVYsCzC2U/*) fails to be parsed due to "Public keys must be 64/66/130 characters in size"/"pubkey string should be 66 or 130 digits long, got: 113".

[apoelstra]

I believe that this is a gap in the definition for DescriptorPublicKey.

It may be that we want to reorganize the type to always have a derivation path (or maybe a vector of derivation paths or something).

The relevant spec is BIP 380 https://github.com/bitcoin/bips/blob/master/bip-0380.mediawiki#user-content-Key_Expressions