Address performance problems in new target selector code

[DRMacIver]

It turns out that:

• We use tag objects as hash keys a lot in target selections
• Our tag objects are all attrs classes and attrs has some significant problems with hashing and equality performance right now: Performance issues with hashing attrs objects python-attrs/attrs#261

This replaces our tags with hash-consed (i.e. memoized) objects, so that two value equal tags are always reference equal. This significantly speeds up their use as keys and set members.

Fixes #919.

[Zac-HD]

Looks good to me - it's pretty obvious what this is doing and why it helps. Take out the unused import of attrs in conjecture/engine.py, and go for it 🎉