Merge #470: Multipath descriptors

ae0f3d9 desc: check for mismatch in the number of derivation paths between multipath keys (Antoine Poinsot)
131bb5b descriptors: BIP389 multipath descriptors support (Antoine Poinsot)
cf401b6 desc keys: add a method to get single-path keys from multipath keys (Antoine Poinsot)
b5eb317 desc keys: unit tests for multipath key expressions (Antoine Poinsot)
34472bb desc keys: implement BIP389 multipath key expressions (Antoine Poinsot)
e122255 desc keys: make 'at_derivation_index' return an error instead of panicing (Antoine Poinsot)
ccd7ef1 desc keys: make parse_xkey_deriv a standalone function (Antoine Poinsot)
fb3cb90 desc keys: rename parse_xkey_origin to parse_key_origin (Antoine Poinsot)
60732fb desc keys: make origin key parsing a standalone function (Antoine Poinsot)

Pull request description:

  This implements [BIP389 multipath descriptors](bitcoin/bips#1354). Fixes #469.

ACKs for top commit:
  sanket1729:
    ACK ae0f3d9.

Tree-SHA512: c7791b02b7bfef964db586eb962289c0058af9a80ad2243cdf4309d6ac1ce65db5cfbfd9b5aa86d733088aeca691449b9777f2d03b969e41ee988d11fe02dd1e