[WIP] Adding mimalloc as an optional malloc override

[hbina]

Related issue https://github.com/JohnSully/KeyDB/issues/179

Edit: messed up the original commit. Ugh

Edit2: This commit is pretty massive should I only include the header & object files required for mimalloc? The first commit adds the repostiory, the second builds the binary required and link with it. Maybe I should have split it to three commits instead. Right now one has to manually build them so I think there should be a way to automatically do this but I am not really good with Makefiles.

Signed-off-by: Hanif Bin Ariffin [email protected]