Move abstract::Policy to crate/abstract/mod.rs

The abstract policy is different from the concrete policy because it is
only part of rust-miniscript and not miniscript in general. To help
differentiate it move it to a separate module.

Note to reviewers, `git` couldn't work out that this is basically a file
move so I've left the `src/policy/abstract.rs` file in the repo but it
is not included in the build. Reviewers can do `diff
src/policy/abstract.rs src/abstract/mod.rs` to ease review. It will be
removed in the next patch.

Author: tcharding

fuzz/fuzz_targets/roundtrip_semantic.rs

@@ -1,9 +1,8 @@
 use std::str::FromStr;
 
 use honggfuzz::fuzz;
-use miniscript::policy;
 
-type Policy = policy::r#abstract::Policy<String>;
+type Policy = miniscript::r#abstract::Policy<String>;
 
 fn do_test(data: &[u8]) {
     let data_str = String::from_utf8_lossy(data);

src/abstract/lift.rs

@@ -15,7 +15,7 @@ use std::error;
 use crate::descriptor::Descriptor;
 use crate::miniscript::{Miniscript, ScriptContext};
 use crate::policy::concrete::Policy as Concrete;
-use crate::policy::r#abstract::Policy as Abstract;
+use crate::r#abstract::Policy as Abstract;
 use crate::sync::Arc;
 use crate::{Error, MiniscriptKey, Terminal};
 
@@ -227,7 +227,7 @@ mod tests {
     use crate::{descriptor::TapTree, Descriptor, Tap};
 
     type ConcretePol = crate::policy::concrete::Policy<String>;
-    type AbstractPol = crate::policy::r#abstract::Policy<String>;
+    type AbstractPol = crate::r#abstract::Policy<String>;
 
     fn concrete_policy_rtt(s: &str) {
         let conc = ConcretePol::from_str(s).unwrap();