Currently, EnvironmentVariablesObject is a strange
holder-that's-not-a-holder. This has implicaitons for things that expect
to get an EnvironmentVariables object, as we can't automatically
unholder it, and instead have to to manually do so. Now we can
automatically unholder it, which makes everything much nicer.
This is useful both from the perspective of optional functionality that
requires a module, and also as I continue to progress with Meson++,
which will probably not implement all of the modules that Meson itself
does.
This makes use of the new convertor and validator arguments, so that we
can check that the languages passed are in fact vaild, and then convert
the native boolean into a MachineChoice internally.