lua-yaml: call __serialize once for all aliases

The YAML format supports aliasing - if the same object is referenced
more than once, it will be encoded in one places with other places
being turned to references:

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

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

This feature is useful for dumping a space list (e.g. box.space)
to the console, because each space is referenced by name and id.

However, it doesn't work if the referenced object implements
the __serialize method:

  tarantool> x = setmetatable({}, {
           >     __serialize = function() return {} end,
           > })
  ---
  ...

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

This happens because we check for aliases in dump_array and dump_table
(with get_yaml_anchor), after calling the __serialize method via
luaL_checkfield. Since the __serialize method may (and usually does)
return a different object on each invocation, aliases aren't detected.

Let's fix it by calling alias detection (get_yaml_anchor) before
luaL_checkfield and passing the anchor to dump_table/dump_array.

Needed for https://github.com/tarantool/tarantool-ee/issues/221
Part of #8240

NO_DOC=bug fix
NO_CHANGELOG=next commit