Use StableHashMap instead of HashMap + BTreeSet for diagnostics

This is a significant reduction in the release mode compile times of
bevy_diagnostics

```
Benchmark #1: touch crates/bevy_diagnostic/src/lib.rs && cargo build --release -p bevy_diagnostic -j1
  Time (mean ± σ):      3.645 s ±  0.009 s    [User: 3.551 s, System: 0.094 s]
  Range (min … max):    3.632 s …  3.658 s    20 runs
```

```
Benchmark #1: touch crates/bevy_diagnostic/src/lib.rs && cargo build --release -p bevy_diagnostic -j1
  Time (mean ± σ):      2.938 s ±  0.012 s    [User: 2.850 s, System: 0.090 s]
  Range (min … max):    2.919 s …  2.969 s    20 runs
```

crates/bevy_diagnostic/src/diagnostic.rs

@@ -1,5 +1,5 @@
-use bevy_utils::{Duration, HashMap, Instant, Uuid};
-use std::collections::{BTreeSet, VecDeque};
+use bevy_utils::{Duration, Instant, StableHashMap, Uuid};
+use std::collections::VecDeque;
 
 /// Unique identifier for a [Diagnostic]
 #[derive(Debug, Copy, Clone, Hash, Eq, PartialEq, PartialOrd, Ord)]
@@ -101,13 +101,13 @@ impl Diagnostic {
 /// A collection of [Diagnostic]s
 #[derive(Debug, Default)]
 pub struct Diagnostics {
-    diagnostics: HashMap<DiagnosticId, Diagnostic>,
-    ordered_diagnostics: BTreeSet<DiagnosticId>,
+    // This uses a [`StableHashMap`] to ensure that the iteration order is deterministic between
+    // runs when all diagnostics are inserted in the same order.
+    diagnostics: StableHashMap<DiagnosticId, Diagnostic>,
 }
 
 impl Diagnostics {
     pub fn add(&mut self, diagnostic: Diagnostic) {
-        self.ordered_diagnostics.insert(diagnostic.id);
         self.diagnostics.insert(diagnostic.id, diagnostic);
     }
 
@@ -134,10 +134,4 @@ impl Diagnostics {
     pub fn iter(&self) -> impl Iterator<Item = &Diagnostic> {
         self.diagnostics.values()
     }
-
-    pub fn ordered_iter(&self) -> impl Iterator<Item = &Diagnostic> {
-        self.ordered_diagnostics
-            .iter()
-            .filter_map(move |k| self.diagnostics.get(k))
-    }
 }

crates/bevy_diagnostic/src/log_diagnostics_plugin.rs

@@ -78,7 +78,7 @@ impl LogDiagnosticsPlugin {
                     Self::log_diagnostic(diagnostic);
                 }
             } else {
-                for diagnostic in diagnostics.ordered_iter() {
+                for diagnostic in diagnostics.iter() {
                     Self::log_diagnostic(diagnostic);
                 }
             }
@@ -96,7 +96,7 @@ impl LogDiagnosticsPlugin {
                     debug!("{:#?}\n", diagnostic);
                 }
             } else {
-                for diagnostic in diagnostics.ordered_iter() {
+                for diagnostic in diagnostics.iter() {
                     debug!("{:#?}\n", diagnostic);
                 }
             }