Add automatic generation of headers

- The 'stripdecls.py' script replaces declarations in all headers by includes to
  generated headers.
  `ag '#\s*if(?!ndef NEOVIM_).*((?!#\s*endif).*\n)*#ifdef INCLUDE_GENERATED'`
  was used for this.
- Add and integrate gendeclarations.lua into the build system to generate the
  required includes.
- Add -Wno-unused-function
- Made a bunch of old-style definitions ANSI

This adds a requirement: all type and structure definitions must be present
before INCLUDE_GENERATED_DECLARATIONS-protected include.

Warning: mch_expandpath (path.h.generated.h) was moved manually. So far it is
the only exception.

src/nvim/api/buffer.c

@@ -20,18 +20,9 @@
 #include "nvim/window.h"
 #include "nvim/undo.h"
 
-static void switch_to_win_for_buf(buf_T *buf,
-                                  win_T **save_curwinp,
-                                  tabpage_T **save_curtabp,
-                                  buf_T **save_curbufp);
-
-static void restore_win_for_buf(win_T *save_curwin,
-                                tabpage_T *save_curtab,
-                                buf_T *save_curbuf);
-
-static void fix_cursor(linenr_T lo, linenr_T hi, linenr_T extra);
-
-static int64_t normalize_index(buf_T *buf, int64_t index);
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "api/buffer.c.generated.h"
+#endif
 
 /// Gets the buffer line count
 ///