compilation of irmin-skeleton is ~25% slower

[samoht]

How to reproduce:

opam switch 4.03+flambda
eval `opam config env`
opam pin -n functoria https://github.com/samoht/functoria.git
opam repo add mirage-dev https://github.com/mirage/mirage-dev.git
opam install mirage
git clone https://github.com/Drup/mirage-skeleton -b functoria
cd mirage-skeleton && make

[mshinwell]

Please try again as per #95