Closures may capture huge objects that user may be unaware.
For example, any closure created by the interpreter contains
a reference to the `Environment`.
@dselsam @kha I don't see many applications for maximizing sharing in
values captured by closures. If we want this feature, we should at
least wrap the `Environment` and `Decl` using external objects
at `mk_stub_closure` at `ir_interpreter.cpp`.
