[plt-scheme] fluid-let-syntax may get flushed

Please drop me a note if you're using `fluid-let-syntax'. My current
plan is to remove it from MzScheme starting with v299.14.

The problem with `fluid-let-syntax' is that it provides a way to break
into a module and access the module's private bindings. We have ignored
the problem up to now, because there were so many other ways to break
into a module. We're closing those other holes without changing the
language much, but I think the solution for `fluid-let-syntax' is
simply to get rid of it.

Thanks,
Matthew