libffi-d: D binding to libffi

[Andrej Mitrovic]

There's a few DLL's floating around for libffi, however none of them
seem to have the symbol "ffi_prep_cif_var" which seems to be needed
for variadic functions.

I can version variadic support out and compile your bindings, the
tests will work fine on win32.
I've also tried compiling libffi on my own but had no success.

I can make a pull with the import lib, the DLL (it's only 99KB), and
the versioned out sections, if you'd like that.