MLton is whole-program, optimizing compiler for Standard ML.
A previous mlton formula simply installed the upstream binary release
and was moved to the boneyard as a binary-only formula (see
Homebrew/homebrew#21780).
This new mlton formula builds from source, using the upstream binary
release to bootstrap.
ClosesHomebrew/homebrew#48694.
Signed-off-by: Andrew Janke <andrew@apjanke.net>