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/digraph.h

@@ -1,12 +1,8 @@
 #ifndef NVIM_DIGRAPH_H
 #define NVIM_DIGRAPH_H
 
-int do_digraph(int c);
-int get_digraph(int cmdline);
-int getdigraph(int char1, int char2, int meta_char);
-void putdigraph(char_u *str);
-void listdigraphs(void);
-char_u *keymap_init(void);
-void ex_loadkeymap(exarg_T *eap);
 
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "digraph.h.generated.h"
+#endif
 #endif  // NVIM_DIGRAPH_H