By using this class we can create ptml::Tags without
XML tags. It is useful in the revng-c part when we
want to generate Plain C. All PTML Tags, from now on,
should be created via this class only.
In addition, port `Yield` library to be using this.
FunctionMetadata was being deserialized every time they were
inspected. This commit introduces a cache structure to prevent this
excessive deserializations.
Djordje Todorovic