Bugfix: mutable serializer & yaml dsl helpers loading depencendies.

[veryrusty]

🎄 ADD: Advent Driven Development 🎄

The easy part:

• require non-core serializers before calling serializer_class->new

The core tries hard to not load YAML.pm when it is not needed. This makes it possible to call the to/from yaml DSL methods without YAML loaded leading to silent serialization failures. Those helpers are also used by the mutable serializer.

• Use deferred subs to load YAML when called as class methods.
• Tests for to_yaml and from_yaml dsl keywords, which also require YAML to be loaded.

Resolves #1568.

[cromedome]

:+1: Nice work!

[cromedome]

Merged! Thanks!