`@as_bool` meta key about koto

[irh]

#154 introduces new boolean coercion rules that mean that only false and null should be treated as false in boolean contexts, and all other values are treated as true. I'm uncertain about the benefit of allowing these rules to be customized with meta logic.

I'd be open to revisiting this with a concrete use case in mind, but for now I'll close this and hope that we can stick to the simple coercion rules.