Add support for building a transcript "owned" by a class (i.e. for static members)
Add explicit context parent (instead of copying attributes)
Minor refactoring
Implement string wrapper for `EventContext.Attributes`
Add new dynamic context example
Refactor transcript context attribute handling and inheritance

* Support three "hierarchies" of contexts:
  * parent;
  * dynamic-parent -- the dynamic-head at time of creation;
  * dynamic-head;
* Properly handle `Attribute` values in `with` methods.
* Properly implement attribute caching in context.