fix large enum variant size difference warning of PermVarAllocation

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.