perf: better derived Hashable for single field structures

[hargoniX]

This PR changes the deriving handler for Hashable to simply call hash x instead of
mixHash 0 (hash x) for single field structures.

Closes: #11818

[hargoniX]

!bench

[leanprover-radar]

Benchmark results for d2b44d6 against b81608d are in! @hargoniX

Warnings (2)

These warnings may indicate that the benchmark results are not directly comparable, for example due to changes in the runner configuration or hardware.

• Runner for run build has different system configurations between commits.
• Runner for run other has different system configurations between commits.

Large changes (1✅)

• ✅ big_do//instructions: -155.0M (-0.5%)

Small changes (280✅, 3🟥)

Too many entries to display here. View the full report on radar instead.