Eta-expansion chapter

[doyougnu]

Chapter on how eta-expansion can change performance.

• should show the core and stg
• need benchmarks for this
• might add it to canonical programs section
• consider describing the oneShot monad trick

References from around the web:

• https://mail.haskell.org/pipermail/haskell-cafe/2022-June/135383.html
• https://discourse.haskell.org/t/memoization-performance/4709