mlton usage message

[Matthew Fluet]

> I would like to simplify the mlton usage message.  My proposal is to move many
> of the switches that are only used by compiler hackers so that they are only
> displayed if the verbosity level is >= 2, so you would only see the switch on a
> usage message if you do "mlton -v2 -z".  I also propose removing some switches.

That sounds good.

> Remove
> --------------------
>     -contify-diag {true|false}         ?specify contify diag
>     -contify {call|cc|cont|dom|none}   ?specify contify strategy

Yup.

The -v2 looks fine, although maybe -native should be at the top.

> I would also like to change -native-ieee-fp to -ieee-fp and change
> -no-polyvariance to -polyvariance {true|false}.

Sounds good.