Refactor monomorphization to use copy_descendants

[acl-cqc]

The new copy_descendants function does much of the hard work copying+substituting nodes in the monomorphization pass. We should simplify the latter to just copy-descendants and then scan for Calls that need further monomorphization.

[acl-cqc]

Note, it might be worth refactoring substitution (the Substitution class and (Type/TypeArg)::subst) to act mutably upon an instance, rather than taking a reference and returning a new instance, first.

That is, add a new fn subst_mut(&mut self, &Substitution) alongside the existing fn subst(&self, s: &Substitution) -> Self EDIT: no need, copy_descendants takes an Option<Substitution>