Update transformations to support generic computational graphs

Many transformations in `transform.jl` and related optimizations in `optimize.jl` apply only to expression trees, i.e., they assume each node has only one parent. Functions `relabel / relabel!` and `standardize_labels / standardize_labels!` are already sufficiently general. All other functions must be corrected to support generic computational graphs, which may include nodes with multiple parents. 