`if method in dir(self):` is very inefficient:
- it must construct a list object listing all the object attributes & methods
- it must then perform a O(N) linear scan of that list
Replace it with the idiomatic `if hasattr(self, method):`, which is a
O(1) expected time hash lookup.
Should fix#11981.
Also making PlayContext a child class of the Playbook Base class,
which gives it access to all of the FieldAttribute code to ensure
field values are correctly typed after post_validation
Fixes#11381
The tags field may contain bad data before it is post_validated, however
some methods assumed it would be a simple list or string. Using itertools
gets us around the problem of the data potentially not being hashable
Fixes#9380
