Refactor trigger decorator and deploy time evaluation logic

[talsperre]

Refactored @trigger logic and simplified the deploy time evaluation logic of @trigger decorator. The current implementation is buggy and fails when running trigger4.py in this gist here.

For testing, refer to all the flows in this gist here. We can add few more tests as well if needed.

Change in contract:

Previously, we allowed deploy time functions (not documented anywhere) at various levels in the @trigger decorator. So something like this was possible:

@trigger(
    events=[
        {
            "name": myfunc1,
            "parameters": [myfunc2, myfunc3]
        },
)
class MyFlow(FlowSpec):

Now, we only allow specifying all of parameters as a deploy time function:

@trigger(
    events=[
        {
            "name": myfunc1,
            "parameters": myfunc2
        },
)
class MyFlow(FlowSpec):

These are also still allowed:

@trigger(events=myfunc)
class MyFlow(FlowSpec):

@trigger(events=[myfunc1, myfunc2])
class MyFlow(FlowSpec):