Don't use serde_json to serialize a simple JSON object

This avoids `rustc_data_structures` depending on `serde_json` which
allows it to be compiled much earlier, unlocking most of rustc.

compiler/rustc_data_structures/src/profiling/tests.rs

@@ -0,0 +1,19 @@
+use super::JsonTimePassesEntry;
+
+#[test]
+fn with_rss() {
+    let entry =
+        JsonTimePassesEntry { pass: "typeck", time: 56.1, start_rss: Some(10), end_rss: Some(20) };
+
+    assert_eq!(entry.to_string(), r#"{"pass":"typeck","time":56.1,"rss_start":10,"rss_end":20}"#)
+}
+
+#[test]
+fn no_rss() {
+    let entry = JsonTimePassesEntry { pass: "typeck", time: 56.1, start_rss: None, end_rss: None };
+
+    assert_eq!(
+        entry.to_string(),
+        r#"{"pass":"typeck","time":56.1,"rss_start":null,"rss_end":null}"#
+    )
+}