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/os/rstream.c

@@ -27,12 +27,9 @@ struct rstream {
   bool reading, free_handle, async;
 };
 
-// Callbacks used by libuv
-static void alloc_cb(uv_handle_t *, size_t, uv_buf_t *);
-static void read_cb(uv_stream_t *, ssize_t, const uv_buf_t *);
-static void fread_idle_cb(uv_idle_t *);
-static void close_cb(uv_handle_t *handle);
-static void emit_read_event(RStream *rstream, bool eof);
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "os/rstream.c.generated.h"
+#endif
 
 /// Creates a new RStream instance. A RStream encapsulates all the boilerplate
 /// necessary for reading from a libuv stream.
@@ -226,6 +223,8 @@ void rstream_read_event(Event event)
   rstream->cb(rstream, rstream->data, event.data.rstream.eof);
 }
 
+// Callbacks used by libuv
+
 // Called by libuv to allocate memory for reading.
 static void alloc_cb(uv_handle_t *handle, size_t suggested, uv_buf_t *buf)
 {