Revamp the symbol lookup mechanism

To complement the future changes in patterns we also want
to allow the execution of patterns to be affected in a similar
manner so that symbol lookup is no longer as confined as it was.

This change adds in the following:

- Symbol lookup by walking through an atoms contained scope/s.
- Better error messaging when symbols are not found.
- Adjusted & new tests (existing ones work).
- Better logging of the symbol lookup mechanism (helpful
  during debugging, although it is very verbose...)

Part of blueprint taskflow-improved-scoping

Change-Id: Id921a4abd9bf2b7b5c5a762337f8e90e8f1fe194
Merge "Revamp the symbol lookup mechanism"