`DocumentError`s are the inteded way of propagating errors from the
pipeline to the frontend when a location is required to inform the user
of the error whereabouts.
This commit reduces build times by introducing a type-erasure layer when
performing a visit on TupleTrees. Basically instead of propagating the
type of the visitor along all of the template castle, we wrap the
visitor into a virtual class with one method for each possible type in
the TupleTree.
This enables a single instatiation of visit algorithm.
Massimo Fioravanti