Convert the assembler phase of the main pipeline in the old format
into a new Pipeline that as the assembler as a stage, where the
input of that stage is the main pipeline. This removes the need of
having "assemblers" as special concepts and thus the corresponding
code in `Pipeline` is removed. The new assembler pipeline is marked
as exported, but the pipeline that builds the tree is not anymore.
Adapt the `describe` and `output` functions of the `v1` format to
handle the assembler pipeline. Also change the tests accordingly.
NB: The id reported for the assembler via `--inspect` and the result
will change as a result of this, since the assembler stage is now
the first and only stage of a new pipeline and thus has no base
anymore.
53e9ec850b