Closure_conversion_aux.EnvUsed to remember which Variable.t values correspond to which Ident.t values during closure conversion, and similarly for static exception identifiers.
val empty : tval add_var : t -> Ident.t -> Variable.t -> tval add_vars : t -> Ident.t list -> Variable.t list -> tval find_var : t -> Ident.t -> Variable.tval find_var_exn : t -> Ident.t -> Variable.tval add_mutable_var : t -> Ident.t -> Mutable_variable.t -> tval find_mutable_var_exn : t -> Ident.t -> Mutable_variable.tval add_static_exception : t -> int -> Static_exception.t -> tval find_static_exception : t -> int -> Static_exception.t