[RFC] Add FOR_ALL_BUFFERS helper

There is `FOR_ALL_WINDOWS` and `FOR_ALL_TAB_WINDOWS` thought `FOR_ALL_BUFFERS` might be handy as well.
Lots of newlines near the top of the file are usually from my #710. Lots of newlines in other places are usually from whatever stripped `#ifdef`s. Specifically here there were lots of macros like

```c
#ifdef FEAT_LUA
    lua_window_free(wp);
#endif
```