[MLton] MLton 20210117 release

I'm pleased to announce a new release of MLton, the whole-program
optimizing compiler for Standard ML, available at
http://mlton.org/Release20210117 (and
https://github.com/MLton/mlton/releases/tag/on-20210117-release).

Relative to MLton 20201002, this is primarily a bug fix release,
fixing a `SimplifyTypes` bug
(https://github.com/MLton/mlton/issues/419).  It also provides
preliminary support for `arm64-darwin` (aka, Apple Silicon) using the
C (or LLVM) codegens.

Binary packages for amd64-linux and amd64-darwin are currently
available from the release page (and are the platforms tested).
Additional binary packages will be made available as they are
contributed by users. We'd like to encourage downstream maintainers
for the various Linux and *BSD distributions to update to the new
release.