Should remove `Display` impl for `semantic::Policy`

[apoelstra]

The Display impl for semantic policies is misleading to users because it resembles a compileable policy (and the objects have very similar names). This, combined with the fact that there's no way to go from a Miniscript back to a compileable policy except by abusing lift, encourages people to do this.

Apparently Miniscript Studio has exposed a "lift" button that produces a semantic policy and then stringifies it into a box labeled "policy" that users can attempt to compile from. This will appear to successfully round-trip in many cases but not others, leading to the impression of bugs.

It may be that people want to present the semantic policies to their users in some way, such as a graphical representation. But presenting it as a flat string that resembless a policy is bound to lead to confusion, and we shouldn't support that.

[shesek]

It would also be useful to add some module-level documentation to miniscript::policy explaining the differences between semantic and concrete policies. It can be pretty confusing.

[apoelstra]

Currently the two policy types are called policy::Concrete (the "real" policy language) and policy::Semantic (the "rust-miniscript abstract representation" one). I propose:

• We rename policy::Concrete to policy::Policy, leaving a type alias to minimize code breakage.
• We move policy::Semantic into its own module so it becomes semantic::Semantic (we can also leave a type alias at the old location but we should deprecate it). We also move the Lift trait, which I think is in policy/mod.rs, to this own module (we can re-export it at the old location, but sadly we cannot deprecate re-exports).
• We stop referring to policy::Semantic as a "semantic policy" or a "policy" at all anywhere in our docs
• We change the Display impl for Semantic to output something goofy, like instead of and(a, b, c) it does (a ∧ b ∧ c). Whoever PRs to do this should provide several examples so we can debate this, especially the threshold notation. (Maybe we want (2 = a + b + c) for thresh(2, a, b, c)?)

Furthermore, I propose adding a method to "uncompile" from a Miniscript to a Policy (or even a Semantic to a Policy), but we should move that to a separate issue because I'm quite conflicted about what to do about percentages.