As stated on the website, duplicate keys make it hard to reason about
malleability as a single signature may unlock multiple paths.
We use a custom KeyCompare function instead of operator< to be explicit
about the requirement.
Parse also key hashes using the Key type. Make this target the first of
the 4 Miniscript fuzz targets in a single `miniscript` file.
Co-authored-by: Pieter Wuille <pieter.wuille@gmail.com>
