lua-yaml: enable aliasing for objects returned by __serialize

The YAML serializer fails to detect aliases in objects returned by
the __serialize method:

tarantool> x = {}
---
...

tarantool> {a = x, b = x}
---
- a: &0 []
  b: *0
...

tarantool> setmetatable({}, {
         >     __serialize = function() return {a = x, b = x} end,
         > })
---
- a: []
  b: []
...

Fix this by scanning the object returned by the __serialize method
(called by luaL_checkfield) for references.

Closes #8240

NO_DOC=bug fix