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)

Cargo.lock

@@ -1336,6 +1336,7 @@ dependencies = [
  "env_logger",
  "image",
  "log",
+ "mimalloc",
  "poll-promise",
  "profiling",
  "puffin",
@@ -1358,6 +1359,7 @@ dependencies = [
  "egui",
  "egui_extras",
  "egui_kittest",
+ "mimalloc",
  "rand",
  "serde",
  "unicode_names2",
@@ -1559,6 +1561,7 @@ dependencies = [
  "emath",
  "epaint_default_fonts",
  "log",
+ "mimalloc",
  "nohash-hasher",
  "parking_lot",
  "profiling",
@@ -2486,6 +2489,16 @@ version = "0.2.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8355be11b20d696c8f18f6cc018c4e372165b1fa8126cef092399c9951984ffa"
 
+[[package]]
+name = "libmimalloc-sys"
+version = "0.1.42"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ec9d6fac27761dabcd4ee73571cdb06b7022dc99089acbe5435691edffaac0f4"
+dependencies = [
+ "cc",
+ "libc",
+]
+
 [[package]]
 name = "libredox"
 version = "0.1.3"
@@ -2591,6 +2604,15 @@ dependencies = [
  "paste",
 ]
 
+[[package]]
+name = "mimalloc"
+version = "0.1.46"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "995942f432bbb4822a7e9c3faa87a695185b0d09273ba85f097b54f4e458f2af"
+dependencies = [
+ "libmimalloc-sys",
+]
+
 [[package]]
 name = "mime"
 version = "0.3.17"