[MLton] mlb support

[Matthew Fluet]

One final issue: there are a few corners cut when Controls.elaborateOnly
is set.  This is set based on Control.warnMatch and Control.keepDefUse,
which in turn is set based on Control.showDefUse and Control.warnUnused.
Hence, if -warn-match false is set on the command line, then no warnings
will be issued, even if elaboration encounters a  warnMatch true
annotation.  I know there is a not insignificant speedup with
elaborateOnly, so I'm inclined to leave the current behavior.  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.