PG_FUNCTION_ARGS

This is used for postgres-compatible functions. The macro is simply:

``` c
// src/include/fmgr.h 
#define PG_FUNCTION_ARGS FunctionCallInfo fcinfo
```

And you'd use it as:

``` c
Datum hello( PG_FUNCTION_ARGS );
```

We just need to define all the structures from C in Rust and we can define the function:

``` rust
extern fn foobar(args: FunctionCallInfo) -> Datum {
    // ...
}
```
