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/sha256.h

@@ -7,11 +7,7 @@ typedef struct {
   char_u buffer[64];
 } context_sha256_T;
 
-void sha256_start(context_sha256_T *ctx);
-void sha256_update(context_sha256_T *ctx, char_u *input, uint32_t length);
-void sha256_finish(context_sha256_T *ctx, char_u digest[32]);
-char_u *sha256_bytes(char_u *buf, int buf_len, char_u *salt, int salt_len);
-int sha256_self_test(void);
-void sha2_seed(char_u *header, int header_len, char_u *salt, int salt_len);
-
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "sha256.h.generated.h"
+#endif
 #endif  // NVIM_SHA256_H