Refactor. Push encountered tag on the stack later after validation.

[Gregable]

Refactor. Up until now, we would, in order:

1. Put the encountered tag on the stack
2. Validate the reference point
3. Update the context with the refence point spec
4. Validate the tag spec
5. Update the context with the tag spec

This refactoring changes that, so now we:

1. Validate the reference point
2. Validate the tag spec
3. Put the encountered tag on the stack (updating parents as needed).
4. Update the context with the refence point spec
5. Update the context with the tag spec

This cleans up many methods in the tag stack, places where code would have state about if the back
of the stack was the node being processed, or the parent node, etc.