functests: Test for error conditions

During testing found the following bugs:

1. msgpack-gen.lua script is completely unprepared for Float values either in 
   return type or in arguments. Specifically:

   1. At the time of writing relevant code FLOAT_OBJ did not exist as well as 
      FLOATING_OBJ, but it would be used by msgpack-gen.lua should return type 
      be Float. I added FLOATING_OBJ macros later because did not know that 
      msgpack-gen.lua uses these _OBJ macros, otherwise it would be FLOAT_OBJ.
   2. msgpack-gen.lua should use .data.floating in place of .data.float. But it 
      did not expect that .data subattribute may have name different from 
      lowercased type name.

2. vim_replace_termcodes returned its argument as-is if it receives an empty 
   string (as well as _vim_id*() functions did). But if something in returned 
   argument lives in an allocated memory such action will cause double free: 
   once when freeing arguments, then when freeing return value. It did not cause 
   problems yet because msgpack bindings return empty string as {NULL, 0} and 
   nothing was actually allocated.
3. New code in msgpack-gen.lua popped arguments in reversed order, making lua 
   bindings’ signatures be different from API ones.

test/functional/api/vim_spec.lua

@@ -219,6 +219,17 @@ describe('api', function()
       eq('\128\253\44', helpers.nvim('replace_termcodes',
                                      '<LeftMouse>', true, true, true))
     end)
+
+    it('does not crash when transforming an empty string', function()
+      -- Actually does not test anything, because current code will use NULL for
+      -- an empty string.
+      --
+      -- Problem here is that if String argument has .data in allocated memory
+      -- then `return str` in vim_replace_termcodes body will make Neovim free
+      -- `str.data` twice: once when freeing arguments, then when freeing return
+      -- value.
+      eq('', meths.replace_termcodes('', true, true, true))
+    end)
   end)
 
   describe('nvim_feedkeys', function()