fix large enum variant size difference warning of PermVarAllocation

by wrapping BranchNumber in an Arc.
PermVarAllocation::Done had size 208 and is now down to 32.
A Box rather than an Arc would be smaller, but it looks like BranchNumber/BranchDesignator are clones a bunch so I expect it to be beneficial to reduce allocations both of the Box itself as well as its content.

src/forms.rs

@@ -25,6 +25,7 @@ use std::fmt;
 use std::hash::{Hash, Hasher};
 use std::ops::{AddAssign, Deref, DerefMut};
 use std::path::PathBuf;
+use std::sync::Arc;
 
 pub type PredicateKey = (Atom, usize); // name, arity.
 
@@ -194,7 +195,7 @@ impl BranchNumber {
 #[derive(Debug)]
 pub enum ChunkedTerms {
     Branch {
-        branch_nums: Vec<BranchNumber>,
+        branch_nums: Vec<Arc<BranchNumber>>,
         arms: Vec<VecDeque<ChunkedTerms>>,
     },
     Chunk {