Sort out serialization of cfg's, or document that it's a nongoal

[anp]

target triple, feature, general cfg, etc.

Maybe this could be configurable in some way? fields in the #[snapshot] annotation?

Could be something like

#[snapshot] // ignores everything, just fails if output doesn't match
#[snapshot(target,some_cfg_name)] // captures target triple in snap, along with an arbitrary cfg value

Would need to have cfg value and things like target triple exposed via macro, and also used as lookup keys in the snap files.