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/api/buffer.h

@@ -6,48 +6,7 @@
 
 #include "nvim/api/private/defs.h"
 
-Integer buffer_get_length(Buffer buffer, Error *err);
-
-String buffer_get_line(Buffer buffer, Integer index, Error *err);
-
-void buffer_set_line(Buffer buffer, Integer index, String line, Error *err);
-
-void buffer_del_line(Buffer buffer, Integer index, Error *err);
-
-StringArray buffer_get_slice(Buffer buffer,
-                             Integer start,
-                             Integer end,
-                             Boolean include_start,
-                             Boolean include_end,
-                             Error *err);
-
-void buffer_set_slice(Buffer buffer,
-                      Integer start,
-                      Integer end,
-                      Boolean include_start,
-                      Boolean include_end,
-                      StringArray replacement,
-                      Error *err);
-
-Object buffer_get_var(Buffer buffer, String name, Error *err);
-
-Object buffer_set_var(Buffer buffer, String name, Object value, Error *err);
-
-Object buffer_get_option(Buffer buffer, String name, Error *err);
-
-void buffer_set_option(Buffer buffer, String name, Object value, Error *err);
-
-Integer buffer_get_number(Buffer buffer, Error *err);
-
-String buffer_get_name(Buffer buffer, Error *err);
-
-void buffer_set_name(Buffer buffer, String name, Error *err);
-
-Boolean buffer_is_valid(Buffer buffer);
-
-void buffer_insert(Buffer buffer, Integer lnum, StringArray lines, Error *err);
-
-Position buffer_get_mark(Buffer buffer, String name, Error *err);
-
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "api/buffer.h.generated.h"
+#endif
 #endif  // NVIM_API_BUFFER_H
-