@nomeata reported on his blog [1] that the ReaderT pattern (i.e. a newtype containing a function) can lead to missed eta-expansions. In his case he reports "Improvement: Allocations: -23.20% Time: -23.00%"

We use this pattern in GHC too so we should check if we could gain anything by forcing eta-expansions in a few places.

MR !3309

Things done:

• !3503 (merged): make the unifier use a one-shot monad.
• !3751 (closed): make the Simplifier use a one-shot monad

[1] https://www.joachim-breitner.de/blog/763-Faster_Winter_5__Eta-Expanding_ReaderT