refactor(lua): use Arena when converting from lua stack to API args

and for return value of nlua_exec/nlua_call_ref, as this uses
the same family of functions.

NB: the handling of luaref:s is a bit of a mess.
add api_luarefs_free_XX functions as a stop-gap as refactoring
luarefs is a can of worms for another PR:s.

as a minor feature/bug-fix, nvim_buf_call and nvim_win_call now preserves
arbitrary return values.

src/nvim/api/private/helpers.h

@@ -34,6 +34,7 @@
 #define CSTR_TO_OBJ(s) STRING_OBJ(cstr_to_string(s))
 #define CSTR_TO_ARENA_STR(arena, s) arena_string(arena, cstr_as_string(s))
 #define CSTR_TO_ARENA_OBJ(arena, s) STRING_OBJ(CSTR_TO_ARENA_STR(arena, s))
+#define CBUF_TO_ARENA_STR(arena, s, len) arena_string(arena, cbuf_as_string((char *)(s), len))
 
 #define BUFFER_OBJ(s) ((Object) { \
     .type = kObjectTypeBuffer, \
@@ -119,6 +120,8 @@
 #define api_init_array = ARRAY_DICT_INIT
 #define api_init_dictionary = ARRAY_DICT_INIT
 
+#define KEYDICT_INIT { 0 }
+
 #define api_free_boolean(value)
 #define api_free_integer(value)
 #define api_free_float(value)