Deprecate or remove Eternal Guard

[stonier]

Pending a decision on sequences w/o memory (see #329), it should be ok to deprecate or remove this idiom as it can easily be instantiated with:

[-] Sequence w/o Memory]
  --> Guard
  [-] Sequence w/ Memory
     --> Task 1
     --> Task 2

[stonier]

Note: #329 needs to be implemented as suggested for this to work.

[stonier]

Decided to keep the decorator, it's a useful / simpler alternative sometimes. The idiom is gone, replaced with an example on how to reproduce the same with no-memory sequences.