Fix for issue #326.

For a foreach (int i, dchar c; str) loop the compiler generates a
delegate with a different type signature then the runtime uses.
The runtime declares the parameters as void* and passes a pointer
to a size_t variable for the index. The code generated by the
compiler expects a ref int. This works on a little endian architecture,
but on a big endian architecture the index is always 0.
This fix changes the parameter type of the passed index to size_t
and generates a cast to the type specified by the user.