Escalation boundary event on a subprocess causes exception

[ekzobrain]

Hi. This code using example file

sources = [File.read("testSettings.bpmn")]
execution = BPMN.new(sources)

causes expection

(ekzo-runner):2:in `<main>': undefined method `attachments' for an instance of BPMN::SubProcess (NoMethodError)

              host_element.attachments << element
                          ^^^^^^^^^^^^

testSettings.bpmn.zip

[ekzobrain]

@jqr Hi. I already fixed this locally and also implemented a lot of other stuff:

1. Complete multi instance support
2. Task local variables and correct input/output mappings
3. Subprocess boundary events (error/escalataion/etc)
4. Error/escalation bubbling
5. Complete link events support
6. Correct scope inheritance
7. Complete support for call activities
8. Some other bugfixes

Review and accept my current PRs, please. They are not so large, but important before I can go on and open PRs for other listed features.

[jqr]

Apologies, I had notifications somehow disabled for this repo, so I'm just now seeing all your stuff. I'll get to this as soon as I can.