[MLton] Added completion to esml-mlb-mode

[Stephen Weeks]

> > I wonder if it would be useful to have MLton spit out all the valid
> > annotations in response to some command-line switch
> 
> I think that this would make sense as a form of self-documentation,

I've added the expert option -show-anns {false|true}.

Currently,

  mlton -show-anns true

prints

  allowFFI {false|true}
  allowOverload {false|true}
  deadCode {false|true}
  forceUsed {false|true}
  nonexhaustiveExnMatch {default|ignore}
  nonexhaustiveMatch {warn|ignore|error}
  redundantMatch {warn|ignore|error}
  sequenceNonUnit {ignore|error|warn}
  warnUnused {false|true}

while

  mlton -expert true -show-anns true

prints

  allowConstant {false|true}
  allowFFI {false|true}
  allowOverload {false|true}
  allowPrim {false|true}
  allowRebindEquals {false|true}
  deadCode {false|true}
  ffiStr {None|Some <longstrid>}
  forceUsed {false|true}
  nonexhaustiveExnMatch {default|ignore}
  nonexhaustiveMatch {warn|ignore|error}
  redundantMatch {warn|ignore|error}
  sequenceNonUnit {ignore|error|warn}
  warnUnused {false|true}