Simplify types

[alex-slynko]

Do we even need to keep the complex recursion for the possible Concourse types?

Maybe it is better to add the limitations where it is only possible to put on_parallel just on the top level and do only once in the on_parallel. This might reduce compilation time and the complexity.

[akshaymankar]

Sometimes it is nice to be able to do something like this:

in_parallel:
- do:
   - task: something
   - task: something
- task: something

Also, it would be required for step hooks.