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/charset.c

@@ -20,12 +20,11 @@
 #include "nvim/os_unix.h"
 #include "nvim/strings.h"
 
-static int win_chartabsize(win_T *wp, char_u *p, colnr_T col);
 
-static int win_nolbr_chartabsize(win_T *wp, char_u *s, colnr_T col,
-                                 int *headp);
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "charset.c.generated.h"
+#endif
 
-static unsigned nr2hex(unsigned c);
 
 static int chartab_initialized = FALSE;