"pre" execute portions of the Ibis tree

[toryhaavik]

I have a use case where I need to intercept normal Ibis execution and operator on multiple Nodes within the tree all at once. This operation will result in a partially-filled scope dictionary for the relevant Nodes. This then short-circuits the depth-first traversal of the tree, because scope has the data it needs to evaluate a given node. A pre_execute dispatcher would allow me to intercept normal execution, do the partial fill of scope and return execution control to the regular dispatcher.