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/hashtab.h

@@ -66,18 +66,7 @@ typedef struct hashtable_S {
   hashitem_T ht_smallarray[HT_INIT_SIZE];      /// initial array
 } hashtab_T;
 
-// hashtab.c
-void hash_init(hashtab_T *ht);
-void hash_clear(hashtab_T *ht);
-void hash_clear_all(hashtab_T *ht, unsigned int off);
-hashitem_T *hash_find(hashtab_T *ht, char_u *key);
-hashitem_T *hash_lookup(hashtab_T *ht, char_u *key, hash_T hash);
-void hash_debug_results(void);
-int hash_add(hashtab_T *ht, char_u *key);
-int hash_add_item(hashtab_T *ht, hashitem_T *hi, char_u *key, hash_T hash);
-void hash_remove(hashtab_T *ht, hashitem_T *hi);
-void hash_lock(hashtab_T *ht);
-void hash_unlock(hashtab_T *ht);
-hash_T hash_hash(char_u *key);
-
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "hashtab.h.generated.h"
+#endif
 #endif  // NVIM_HASHTAB_H