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/os/fs.c

@@ -8,8 +8,9 @@
 #include "nvim/path.h"
 #include "nvim/strings.h"
 
-static bool is_executable(const char_u *name);
-static bool is_executable_in_path(const char_u *name);
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "os/fs.c.generated.h"
+#endif
 
 // Many fs functions from libuv return that value on success.
 static const int kLibuvSuccess = 0;
@@ -349,4 +350,3 @@ bool os_file_info_id_equal(FileInfo *file_info_1, FileInfo *file_info_2)
   return file_info_1->stat.st_ino == file_info_2->stat.st_ino
          && file_info_1->stat.st_dev == file_info_2->stat.st_dev;
 }
-