AST Alternative Memory Representations

The types for Policy are kind of annoying because they are "overgeneric". E.g., if I want a a vec of PKs, I have to
first get a vec of keys and then map Policy::Pk over it to convert it to the correct type, and I can't directly cast a Vec to a Vec. And if I did just do a Vec, it's over generic since it could contain a locktime.

It would be really cool if there was a way to be able to compile time limit the polymorphism of the Policy branches so that the
conversions from Pk to Policy::Pk could be done in place.

I'm not sure how to actually solve for this cleanly.

This also applies to miniscript, and there could be some advantages to being able to make homogeneous but cost free vecs in the translation layer.

Start by tracing the Policy and miniscript AST type definitions and the translation layer mentioned in the issue. Compare possible compile-time-restricted representations for Policy branches and homogeneous vectors; done requires an agreed design that supports the requested conversions without unnecessary cost.