18% faster build time when using DMD with mimalloc!

[ryuukk_]

Simply just compiling my project with that allocator makes it 18% 
faster

 From an average of 450ms down to 381ms, fastest was 374ms

```
$ hyperfine "make build-game" "make build-game-mimalloc"
Benchmark 1: make build-game
   Time (mean ± σ):     450.4 ms ±   4.6 ms    [User: 383.8 ms, 
System: 64.7 ms]
   Range (min … max):   443.9 ms … 458.9 ms    10 runs

Benchmark 2: make build-game-mimalloc
   Time (mean ± σ):     381.7 ms ±   6.7 ms    [User: 346.2 ms, 
System: 34.1 ms]
   Range (min … max):   374.7 ms … 393.4 ms    10 runs

Summary
   make build-game-mimalloc ran
     1.18 ± 0.02 times faster than make build-game
```

Try with your project, and share the result here, and perhaps we 
should make it the default if it's an improvement for everyone?


`LD_PRELOAD=/usr/lib/libmimalloc.so dmd`