Variable declarations should only be considered equal if their identity
is equal. In other words, just because two different variable
declarations share the same name, they aren’t automatically equal. This
is to support situations in which nested variable declarations shadow
variable declarations of the same name at a higher level.
These fields may have to be shared in other data structures, such as a
variable declaration stack. In order to accomplish that, it’s necessary
to wrap the variable declarations in std::rc::Rc.
This replaces the unordered sets used for function and predicate
declarations with ordered sets. This is useful when these declarations
are supposed to be processed in a stable and meaningful order.
