[MLton] mlb support

[Stephen Weeks]

> On the other hand, there could be situation where you would not see
> a warning when compiling with -warn-match false -stop tc and would
> see the warning when compiling with just -warn-match false.

Perhaps we need two flags

  -enable-warn-match {false|true}
	Governs whether there can be match warnings at all.

  -warn-match {false|true}
	The default warnMatch annotation for mlbs.

Then, we would set elaborateOnly as

  elaborateOnly := stop = Place.TypeCheck
                   andalso not (!enableWarnMatch)
                   andalso not (!keepDefUse)

Match warnings would only be generated for some code only if both
enableWarnMatch was true and the code's warnMatch annotation was true.


Or perhaps we should just add a new -stop, "elab".