[MLton] MLB export filters and -show-def-use

[Matthew Fluet]

> Another, well, inconvenience I noticed while working on the def-use mode is
> that (non-renaming) MLB export filters partition the def-use information in
> an inconvenient way. 

Yes, I ran into this when playing with the def-use mode.

 > Perhaps a
> better way to handle non-renaming export filters would be to treat them as
> plain uses (and not as definitions).

We currently expand the export filter name to an export filter binding 
by the parser, so either the grammar needs to be changed or we change 
the elaborator to recognize when the lhs and rhs are the same symbol.