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/version.h

@@ -1,12 +1,7 @@
 #ifndef NVIM_VERSION_H
 #define NVIM_VERSION_H
 
-int highest_patch(void);
-int has_patch(int n);
-void ex_version(exarg_T *eap);
-void list_version(void);
-void maybe_intro_message(void);
-void intro_message(int colon);
-void ex_intro(exarg_T *eap);
-
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "version.h.generated.h"
+#endif
 #endif  // NVIM_VERSION_H