Added define_fn

- updated write_fn_step to support attributes, bind names and
  explicit argument types
- added define_fn to generate write_fn_step sequences
- updated concatenate to define_fn as an example

src/foreign_macros/mod.rs

@@ -2,5 +2,6 @@ mod atomic_defaults;
 mod atomic_impl;
 mod atomic_inert;
 mod atomic_redirect;
+mod define_fn;
 mod externfn_impl;
 mod write_fn_step;