Use Instant over SystemTime to record timings

Closes #3575.

crates/compiler/load_internal/src/wasm_instant.rs

@@ -0,0 +1,42 @@
+#![cfg(target_family = "wasm")]
+/*
+For the Web REPL (repl_www), we build the compiler as a Wasm module.
+Instant is the only thing in the compiler that would need a special implementation for this.
+There is a WASI implementation for it, but we are targeting the browser, not WASI!
+It's possible to write browser versions of WASI's low-level ABI but we'd rather avoid it.
+Instead we use these dummy implementations, which should just disappear at compile time.
+*/
+
+#[derive(Debug, Clone, Copy)]
+pub struct Instant;
+
+impl Instant {
+    pub fn now() -> Self {
+        Instant
+    }
+    pub fn duration_since(&self, _: Instant) -> Duration {
+        Duration
+    }
+    pub fn elapsed(&self) -> Duration {
+        Duration
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+pub struct Duration;
+
+impl Duration {
+    pub fn checked_sub(&self, _: Duration) -> Option<Duration> {
+        Some(Duration)
+    }
+}
+
+impl Default for Duration {
+    fn default() -> Self {
+        Duration
+    }
+}
+
+impl std::ops::AddAssign for Duration {
+    fn add_assign(&mut self, _: Duration) {}
+}