[plt-scheme] mzscheme subdirectory for SWIG

 >> I think we should probably convert the SWIG stuff to FFI. I added
 >> the struct code that is currently in SWIG. If you define a struct
 >> mapping, then SWIG will create a scheme structure if your func is
 >> supposed to return a struct. It's fairly limited since it only goes
 >> one way (C->scheme), but it suited my purposes.
 >
 >FWIW, the foreign code will keep C structs as C structs, so you have
 >no overhead for keeping them around.  You inly translate values when
 >you access the c struct object.

I realise that :) That is why I really would like to see the FFI in play
in the mzscheme module. I'll have to wait till v300 though before I
start playing with it. We are running a production env based on v208, so
I am reluctant to just pull the rug from under it and go for 299. 

--Dima