Use mimalloc as global memory allocator (#749)

This should make allocation behavior more consistent across different
builds and reduce memory fragmentation issues.

The allocation behavior of the `nativelink` executable is now
configurable with `MIMALLOC_*` environment variables as described here:

https://microsoft.github.io/mimalloc/environment.html

BUILD.bazel

@@ -27,6 +27,7 @@ rust_binary(
         "@crates//:console-subscriber",
         "@crates//:futures",
         "@crates//:hyper",
+        "@crates//:mimalloc",
         "@crates//:parking_lot",
         "@crates//:prometheus-client",
         "@crates//:rustls-pemfile",