Changes global allocator to mimalloc, improving performance. (#9415)

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this PR makes nushell use mimalloc as the default allocator, this has
the benefit of reducing startup time on my machine. `17%` on linux and
`22%` on windows, when testing using hyperfine.
the overhead to compile seem to be quite small, aswell as the increase
of binary size quite small
on linux the binary went from `33.1mb` to `33.2mb`

linux

![image](https://github.com/nushell/nushell/assets/17986183/ba5379b4-2c08-483a-a9ff-a9d8524d2943)

windows 

![image](https://github.com/nushell/nushell/assets/17986183/fda5090f-96a9-48d1-ada4-617694b9d880)


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crates/nu-cmd-lang/Cargo.toml

@@ -28,6 +28,7 @@ ahash = "0.8.3"
 shadow-rs = { version = "0.22", default-features = false }
 
 [features]
+mimalloc = []
 which-support = []
 trash-support = []
 sqlite = []

crates/nu-cmd-lang/src/core_commands/version.rs

@@ -117,6 +117,12 @@ pub fn version(
         vals.push(Value::string(build_rust_channel, call.head));
     }
 
+    cols.push("allocator".to_string());
+    vals.push(Value::String {
+        val: global_allocator().to_string(),
+        span: call.head,
+    });
+
     cols.push("features".to_string());
     vals.push(Value::String {
         val: features_enabled().join(", "),
@@ -144,6 +150,14 @@ pub fn version(
     .into_pipeline_data())
 }
 
+fn global_allocator() -> &'static str {
+    if cfg!(feature = "mimalloc") {
+        "mimalloc"
+    } else {
+        "standard"
+    }
+}
+
 fn features_enabled() -> Vec<String> {
     let mut names = vec!["default".to_string()];