Remove/Refactor tags

[billyquith]

TagHolder is the base class for all declared types. This is a specialised use of the types and seems like it should be in the client code. Each TagHolder contains a std::map, which is also an unwanted overhead.

Perhaps this could be removed and custom user data attached to objects?

I'll remove this for now and rethink it later.

[billyquith]

We'll add a common base class ponder::Type for now.

[billyquith]

In dev branch.

[billyquith]

See #98 for mechanism for this.