Miscellaneous improvements/changes

[imthatgin]

• Addition of donburi.AllComponentTypes() which returns all component types created using donburi.NewComponentType()
• Changed the Tag feature from a struct{} to a string, as struct{} was hard to check for in various things, and tags can now be named when creating them.

[yohamta]

@imthatgin LGTM, thank you so much for the improvement!