RFC: improving performance of `deriving Hashable` on single-field structures

Proposal

The hash produced by deriving Hashable on a single-field structure should be the same as the hash of the field type.

Currently, these two types have very different hashes, where the structure hash has a large slowdown compared to the other one:

abbrev T := Nat

structure U where
  value: Nat
deriving Hashable

On our benchmarks that rely a lot on hashmaps, going from U to T lead to a 2x slowdown overall, whereas the two types are representing exactly the same thing.

Manually deriving the Hashable removes that slowdown, and I believe this implementation should be the one generated by deriving Hashable

instance : Hashable U where
  hash u := hash u.value

Impact

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No files, tests, or entry points are named. Start by locating the deriving Hashable implementation and relevant benchmarks, then determine how single-field structures are handled; done means the generated hash matches the field hash without regressing other derived instances.