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.

src/nvim/api/vim.c

@@ -139,7 +139,7 @@ String nvim_replace_termcodes(String str, Boolean from_part, Boolean do_lt,
 {
   if (str.size == 0) {
     // Empty string
-    return str;
+    return (String) { .data = NULL, .size = 0 };
   }
 
   char *ptr = NULL;
@@ -843,7 +843,7 @@ static void write_msg(String message, bool to_err)
 /// @return its argument.
 Object _vim_id(Object obj)
 {
-  return obj;
+  return copy_object(obj);
 }
 
 /// Returns array given as argument
@@ -856,7 +856,7 @@ Object _vim_id(Object obj)
 /// @return its argument.
 Array _vim_id_array(Array arr)
 {
-  return arr;
+  return copy_object(ARRAY_OBJ(arr)).data.array;
 }
 
 /// Returns dictionary given as argument
@@ -869,5 +869,18 @@ Array _vim_id_array(Array arr)
 /// @return its argument.
 Dictionary _vim_id_dictionary(Dictionary dct)
 {
-  return dct;
+  return copy_object(DICTIONARY_OBJ(dct)).data.dictionary;
+}
+
+/// Returns floating-point value given as argument
+///
+/// This API function is used for testing. One should not rely on its presence
+/// in plugins.
+///
+/// @param[in]  flt  Value to return.
+///
+/// @return its argument.
+Float _vim_id_float(Float flt)
+{
+  return flt;
 }