Merge rust-bitcoin/rust-miniscript#470: Multipath descriptors

ae0f3d95fa408bfc5e535050743a0f0ed8cb2963 desc: check for mismatch in the number of derivation paths between multipath keys (Antoine Poinsot)
131bb5bcfd54ff0c178fb90460b733bcf06751c1 descriptors: BIP389 multipath descriptors support (Antoine Poinsot)
cf401b602846f392af3dc88e1be8509c17d01cef desc keys: add a method to get single-path keys from multipath keys (Antoine Poinsot)
b5eb3175292e567cd31590102fc1c92f874c4a85 desc keys: unit tests for multipath key expressions (Antoine Poinsot)
34472bb6a9f820dc9057cbb4ea71caf9ecd8a6bc desc keys: implement BIP389 multipath key expressions (Antoine Poinsot)
e122255e3bab25395c4c5d0f25107ed5fc80dd15 desc keys: make 'at_derivation_index' return an error instead of panicing (Antoine Poinsot)
ccd7ef17eb036566c40ab45bc7daed6656221cd2 desc keys: make parse_xkey_deriv a standalone function (Antoine Poinsot)
fb3cb9094ac6b5a1afa4ea3980b941abdea6a316 desc keys: rename parse_xkey_origin to parse_key_origin (Antoine Poinsot)
60732fb37ca14dad8b1eff9301cb1bd9cadf8a72 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 ae0f3d95fa408bfc5e535050743a0f0ed8cb2963.

Tree-SHA512: c7791b02b7bfef964db586eb962289c0058af9a80ad2243cdf4309d6ac1ce65db5cfbfd9b5aa86d733088aeca691449b9777f2d03b969e41ee988d11fe02dd1e

Author: heap-coder