Common type idiom + sizeof/1 as library function

We can now correctly highlight custom types suffixed with `_t`, a common idiom in C. The syntax highlighter GitHub uses already does it right:

``` C
custom_type_t variable = {};
```

Additionally, the `sizeof` operator is treated different than the `sizeof/1` function. However, the `sizeof/1` function is now recognized as a library function (and thus not user function), for correct syntax highlighting.