Use mimalloc for benchmarks (#7029)

`mimalloc` is a _much_ faster allocator, especially important when doing
a lot of small allocations (which egui does).

We use `mimalloc` in Rerun, and I recommend everyone to use it.

## The difference it makes

![image](https://github.com/user-attachments/assets/b22e0025-bc5e-4b3c-94e0-74ce46e86f85)

crates/egui_demo_app/Cargo.toml

@@ -86,6 +86,7 @@ env_logger = { version = "0.10", default-features = false, features = [
   "auto-color",
   "humantime",
 ] }
+mimalloc.workspace = true
 rfd = { version = "0.15.3", optional = true }
 
 # web:

crates/egui_demo_app/src/main.rs

@@ -4,6 +4,9 @@
 #![allow(rustdoc::missing_crate_level_docs)] // it's an example
 #![allow(clippy::never_loop)] // False positive
 
+#[global_allocator]
+static GLOBAL: mimalloc::MiMalloc = mimalloc::MiMalloc; // Much faster allocator, can give 20% speedups: https://github.com/emilk/egui/pull/7029
+
 // When compiling natively:
 fn main() -> eframe::Result {
     for arg in std::env::args().skip(1) {