Since we're no longer using storage keys to refer to elements and
regulators, we don't need to store these items in keyed collections
anymore.
To keep element and regulator pointers in `BTreeSet` collections, we
implement `Ord` for `Serial` trait objects. As a bonus, this lets us
turn the element-wise regulator collections back into `BTreeSet`.
In the process, move the code that used to handle serial numbering for
elements into the `Serial` trait, where it can provide serial numbers
for regulators too.
Use reference-counted pointers, rather than collection keys, to refer to
elements in tasks like specifying the subjects of regulators, handling
selection, and creating interface components.
Rust pointers don't seem to be designed for use as identifiers, so it's
probably more idiomatic and possibly more reliable to manage our own
unique identifier system.
https://stackoverflow.com/a/72149089
This also lets us get rid of the `ElementRc` newtype.
