Problem: BPXE.Engine.BPMN API ergonomics

[yrashk]

This particularly applies to add_node function. It would have been great to have these:

• [x] Passing attributes as maps or keyword lists. Will greatly reduce %{} noise in tests. (done in ed3c532e1989c00d63f432b62cffb4385b7f1d90)
• [x] Detecting if no attributes were supplied but a body was (very useful for simplifying programmatic use of script, incoming, outgoing, etc. -- currently these require use of an empty map, which isn't great) (done in 8e31d40e387ff6a13eb0bc19fb5e8028ed3081b7)
• [ ] Improved naming for add_node/complete_node
• [ ] Validation of string attributes against the schema (one can dream?)