Store elements and regulators without keys

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`.

app-proto/Cargo.lock

@@ -1,6 +1,6 @@
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
-version = 3
+version = 4
 
 [[package]]
 name = "ahash"
@@ -90,7 +90,6 @@ dependencies = [
  "nalgebra",
  "readonly",
  "rustc-hash",
- "slab",
  "sycamore",
  "wasm-bindgen-test",
  "web-sys",
@@ -414,15 +413,6 @@ dependencies = [
  "wide",
 ]
 
-[[package]]
-name = "slab"
-version = "0.4.9"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8f92a496fb766b417c996b9c5e57daf2f7ad3b0bebe1ccfca4856390e3d3bb67"
-dependencies = [
- "autocfg",
-]
-
 [[package]]
 name = "slotmap"
 version = "1.0.7"