App: store selection in hash map

Switch `Assembly.elements` to a hash map too, since that's probably
closer to what we'll want in the future.

app-proto/sketch-outline/src/assembly.rs

@@ -1,4 +1,5 @@
 use nalgebra::DVector;
+use rustc_hash::FxHashMap;
 use sycamore::reactive::Signal;
 
 #[derive(Clone, PartialEq)]
@@ -6,16 +7,12 @@ pub struct Element {
     pub id: String,
     pub label: String,
     pub color: [f32; 3],
-    pub rep: DVector<f64>,
-    
-    /* TO DO */
-    // does this belong in the element data?
-    pub selected: Signal<bool>
+    pub rep: DVector<f64>
 }
 
 // a complete, view-independent description of an assembly
 #[derive(Clone)]
 pub struct Assembly {
     // the order of the elements is arbitrary, and it could change at any time
-    pub elements: Signal<Vec<Element>>
+    pub elements: Signal<FxHashMap<String, Element>>
 }