Zero destination address when setting ffi arguments

It makes the arguments for ffi call more reasonable.
change cdef syntax to use long string

previous cdef syntax:

```
cdef("int foo();")
```

new syntax:

```
cdef[[
int foo();
]]
```

Signed-off-by: Qingping Hou <qingping.hou@gmail.com>
Signed-off-by: Yicheng Qin <qycqycqycqycqyc@gmail.com>
update FFI samples/tests/document to use new cdef syntax

Signed-off-by: Qingping Hou <qingping.hou@gmail.com>
Signed-off-by: Yicheng Qin <qycqycqycqycqyc@gmail.com>