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/server.c

@@ -42,9 +42,9 @@ typedef struct {
 
 static PMap(cstr_t) *servers = NULL;
 
-static void connection_cb(uv_stream_t *server, int status);
-static void free_client(uv_handle_t *handle);
-static void free_server(uv_handle_t *handle);
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "os/server.c.generated.h"
+#endif
 
 /// Initializes the module
 void server_init()