Keystone passives should allow weapon set allocation

This may or may not be a little broken or hard to balance, but the inability to do so really just feels like a 'no fun allowed' bandaid.

I'm playing with a crossbow shield build and I 'realised' that I could take Iron Reflexes (conv all evasion to armour) for my shield weapon set to adapt my defenses on the fly. I was disappointed when I couldn't actually do this.

Weapon set keystones has huge potential for application. I would love to see it.