[plt-scheme] silex vs. ffi

Oh yeah, a parser-based tool becomes more useful for generating a 
starting-point for bindings if you're dealing with a *large* API, and 
can specialize your tool for the conventions of that API.

For example, you might know as human knowledge that a certain 
application-specific type, or one prefixed with a certain CPP macro, 
should always be passed in a certain way.  So you can hard-code that 
application-specific special case into your generator.