cl-interpret: Builtin refactor

- Everything is different now
  - Builtins are now built on top of Rust functions, so they can be recursive!
  - TODO: allow macro-defined builtins to call each other?
- The builtins! macro is a lot nicer to work with
  - No redundant return value
  - Maps the result over Into::into, allowing for type inference!
  - Uses explicit pattern syntax instead of weird binding, where possible
  - Does not #[allow(unused)], so you'll get unused variable warnings now!

compiler/cl-interpret/src/interpret.rs

@@ -345,7 +345,8 @@ mod assignment {
 
     pub(super) fn pat_assign(env: &mut Environment, pat: &Pattern, value: ConValue) -> IResult<()> {
         let mut substitution = HashMap::new();
-        append_sub(&mut substitution, pat, value).map_err(|_| Error::PatFailed(pat.clone()))?;
+        append_sub(&mut substitution, pat, value)
+            .map_err(|_| Error::PatFailed(pat.clone().into()))?;
         for (path, value) in substitution {
             assign_path(env, path, value)?;
         }