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/tabpage.h

@@ -6,13 +6,7 @@
 
 #include "nvim/api/private/defs.h"
 
-Object tabpage_get_var(Tabpage tabpage, String name, Error *err);
-
-Object tabpage_set_var(Tabpage tabpage, String name, Object value, Error *err);
-
-Window tabpage_get_window(Tabpage tabpage, Error *err);
-
-Boolean tabpage_is_valid(Tabpage tabpage);
-
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "api/tabpage.h.generated.h"
+#endif
 #endif  // NVIM_API_TABPAGE_H
-