Refactors MERGE to be handled by as an UpdateAction instead of as a Pipe
Allow MERGE to run inside FOREACH
Fix problems with CREATE for relationships without declared nodes

When doing a CREATE with a relationship where the end nodes
have not already been declared, the old code would add node
creation to the execution plan. This was, however, buggy, and
for some situations, especially surrounding FOREACH, the code
ended up creating much too many elements.

Fixes error handling of erroneously provided lush nodes.

Also changes all bare fields into methods.
Verify that pretty graph doc tests update the graph correctly
(i.e. create the correct number of nodes, relationships, etc.)