Add support for extracting single-path policies from a multi-path policy

[shesek]

Keys and descriptors already support this in the Minsc runtime, using the singles() function or array-like index access.

This was implemented using rust-miniscript's into_single_keys()/into_single_descriptors(), which doesn't have an equivalent for policies.

Adding support for policies is pretty straightforward, but not implemented yet. PRs welcome :)