Automagical external function header definition

At the moment we describe arguument types via `$lb()` call, i.e.

```
set argTypes = $lb(..gd.#POINTER, ..gd.#POINTER, ..gd.#INT, ..gd.#INT)
do ..gd.CallFunction("gdImageGifAnimBegin", ..gd.#VOID, argTypes, im, out, 1, 0)
```

It would be cool if we would have some _compile-time_ way to define function argument types, so type coercion will be done authomagically at the call moment.
