Add FFI module example.

examples/unix/ffi_example.py

+import ffi
+
+libc = ffi.open("libc.so.6")
+print("libc:", libc)
+print()
+
+# Declare few functions
+perror = libc.func("v", "perror", ["s"])
+time = libc.func("i", "time", "p")
+open = libc.func("i", "open", ["s", "i"])
+qsort = libc.func("v", "qsort", "piip")
+# And one variable
+errno = libc.var("i", "errno")
+
+print("time:", time)
+print("UNIX time is:", time(None))
+print()
+
+perror("ffi before error")
+open("somethingnonexistent__", 0)
+print(errno)
+perror("ffi after error")
+print()
+
+def cmp(pa, pb):
+    a = ffi.as_bytearray(pa, 1)
+    b = ffi.as_bytearray(pb, 1)
+    print("cmp:", a, b)
+    return a[0] - b[0]
+
+cmp_c = ffi.callback("i", cmp, "pp")
+print("callback:", cmp_c)
+
+# TODO: violates Py semantics, pass bytearray
+s = "foobar"
+print("org string:", s)
+qsort(s, len(s), 1, cmp_c)
+print("qsort'ed:", s)