0.13.0

Breaking change: adopt the new Durability API proposed in RFC #6
- this replaces and generalizes the existing concepts of constants
Breaking change: remove "volatile" queries
- instead, create a normal query which invokes the report_untracked_read method on the salsa runtime
introduce "slots", an optimization to salsa's internal workings
document #[salsa::requires] attribute, which permits private dependencies
Adopt AtomicU64 for runtimeId (#182)
use ptr::eq and ptr::hash for readability
upgrade parking lot, rand dependencies

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