Add a sub-command to compile a miniscript policy into a descriptor

[notmandatory]

Given a miniscript spending "policy" and an optional script type ("sh", "wsh", or "sh-wsh") with default "wsh", compile the policy into it's corresponding output descriptor.

This is a command we found would be handy to have while creating the "Spending Policy Demo" blog post.

[afilini]

Fixed by #21