Execution blocking can be done by returning an `ExecutionBlocker`
(imported from graph_utils) either in place of results or as a specific
output. Any node that uses an `ExecutionBlocker` as input will be
skipped. This operates on a per-entry basis when inputs are lists.
If the `ExecutionBlocker` is initialized with an error message, that
message will be displayed on the first node it's used on (and further
downstream nodes will be silently skipped).
Note that For loops still require WAS-ns. They just expand to using the
integer operation node internally.
Also adds some nodes for accumulation operations.
