Use mimalloc memory allocator

[atlv24]

Use https://github.com/microsoft/mimalloc for better memory allocation performance and lower total memory usage.

[ashalkhakov]

Hi! There are multiple ways to go about this. Which one do you think of?

• we can replace the libc malloc (say) with mimalloc (basically what the memalloc authors describe in their README file)
• we can use the 'user' memory allocation in the runtime (https://github.com/githwxi/ATS-Postiats/blob/master/ccomp/runtime/pats_ccomp_memalloc.h#99 -- use -DATS_MEMALLOC_USER flag during compilation, link against mimalloc, and use mimalloc to implement the corresponding routines, basically what they describe in their README)
• we can provide bindings to the mimalloc library so it can be used alongside with the existing allocator

[atlv24]

I had the first one in mind, but I would like to profile before and after tests on a variety of cases first. I mostly put this issue here to make sure there was awareness of this lib, and to remember to play around with an integration in about 1-2 weeks when i have more free time. By the way, this project is awesome, im looking forward to using it.