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.

scripts/msgpack-gen.lua

@@ -94,7 +94,9 @@ output:write([[
 ]])
 
 for i = 1, #headers do
-  output:write('\n#include "nvim/'..headers[i]..'"')
+  if headers[i]:sub(-12) ~= '.generated.h' then
+    output:write('\n#include "nvim/'..headers[i]..'"')
+  end
 end
 
 output:write([[