Introduce a second text format for pipelines/plans

[jacques-n]

Right now pipelines are kind of hard to read if there are nodes that have multiple inputs. Let's add a second supported pipeline pattern that is more similar to other query plans. It can use root to leaf ordering. In the design, it will list out a straight pipeline one line after another. If an operator has more than one input, it's children are indented (and subsequent as well) but are prefixed with a dash. For example, a plan that was a two tables read followed each followed by a filter joined together and then aggregated would like this in the old format and the new format:

Old:

pipelines {
  read -> filter -> join
  read2 -> filter2 -> join
  join -> aggregate -> root
}

New Format:

plan {
  aggregate
  join
  -filter
   read
  -filter2
   read2
}

I propose that both formats are supported but only one can be used at a time. When printing, we can request which we want.