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/private/helpers.h

@@ -13,32 +13,7 @@
     err->set = true;                               \
   } while (0)
 
-void try_start(void);
-
-bool try_end(Error *err);
-
-Object dict_get_value(dict_T *dict, String key, Error *err);
-
-Object dict_set_value(dict_T *dict, String key, Object value, Error *err);
-
-Object get_option_from(void *from, int type, String name, Error *err);
-
-void set_option_to(void *to, int type, String name, Object value, Error *err);
-
-Object vim_to_object(typval_T *obj);
-
-buf_T *find_buffer(Buffer buffer, Error *err);
-
-win_T * find_window(Window window, Error *err);
-
-tabpage_T * find_tab(Tabpage tabpage, Error *err);
-
-/// Copies a C string into a String (binary safe string, characters + length)
-///
-/// @param str the C string to copy
-/// @return the resulting String, if the input string was NULL, then an
-///         empty String is returned
-String cstr_to_string(const char *str);
-
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "api/private/helpers.h.generated.h"
+#endif
 #endif  // NVIM_API_PRIVATE_HELPERS_H
-