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.c

@@ -27,9 +27,10 @@ typedef struct digraph {
   result_T result;
 } digr_T;
 
-static int getexactdigraph(int char1, int char2, int meta_char);
-static void printdigraph(digr_T *dp);
 
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "digraph.c.generated.h"
+#endif
 // digraphs added by the user
 static garray_T user_digraphs = {0, 0, (int)sizeof(digr_T), 10, NULL};
 
@@ -1721,7 +1722,6 @@ typedef struct {
 
 #define KMAP_MAXLEN 20  // maximum length of "from" or "to"
 
-static void keymap_unload(void);
 
 /// Set up key mapping tables for the 'keymap' option.
 ///