[MLton] mlb annotations

[Matthew Fluet]

When I added support for quoted (") filenames in mlbs, it occured to me
that it might be useful to make annotations be quoted as well.

Some pros:
  + unify "parsing" of annotations from command line and from .mlbs
  + simpler grammar; it's a bit ad hoc what the grammar currently checks.
    it enforces annotations as "comma delimited lists of (whitespace
    delimited lists of identifiers)", but does nothing to check the
    validity of the annotations until elaboration.  This was a deliberate
    decision on my part, as I wanted it to be clear that annotations
    were an open ended feature.  It also meant that we could parse
    some other system's annotations.  Choosing string constants
    as the representation of annotations keeps these design decisions.

Some cons:
  + cumbersome to write multi-line annotations:

ann
  sequenceUnit false,
  warnMatch true,
  warnUnused true, forceUsed
in
  ...
end

would be written either

ann
  "sequenceUnit false, warnMatch true, warnUnused true, forceUsed"
in
  ...
end

or

ann
  "sequenceUnit false, \
   warnMatch true, \
   warnUnused true, forceUsed"
in
  ...
end