[MLton] more edits to flatten.fun

[Lukasz S Ziarek]

There is one problem that still needs to be addressed in flatten and that
is the case of tupples within other tupples.

Something like: ((a,b),(d,e,(f,g)),h,i,j)

I believe the best solution to this, is in the flatten all case, is to
simply have a function that will recurse through the Var.t vector of a
tuple as well as the type and if we see another tupple type mark that as
flat. Ofcourse this will require a change in doitArgs and potentially
doitCaseCon. Currently flatten.fun does not do anything with nested
tuples, correct? Will any of these changes need to be propogated to local
flatten?

Luke