There's a closed pull request on the void-packages repository

New Package: mlton-20210117
https://github.com/void-linux/void-packages/pull/44621

Description:
#### Testing the changes
- I tested the changes in this PR: **YES**


#### New package
- This new package conforms to the [package requirements](https://github.com/void-linux/void-packages/blob/master/CONTRIBUTING.md#package-requirements): **YES**

#### Local build testing
- I built this PR locally for my native architecture, x86_64

MAJOR NOTE: Similar to openjdk mlton needs to be built with a binary of itself. These binaries are only available for x86_64. Cross compile should be completely possible but compilation on platforms other than x86_64 is not possible unless a binary is made for that arch. Comments and ideas welcome and needed as this is why the archs are restricted to x86_64.