put in the value restriction

[jeske]

Sam says it's a bug that the value restriction is not in the new irken compiler.

https://realworldocaml.org/v1/en/html/imperative-programming-1.html#the-value-restriction

Apparently it's important not to use the MLton manual, because it has an incorrect description of the value restriction.

https://cstheory.stackexchange.com/questions/38173/why-does-mls-value-restriction-stop-you-from-capturing-parametric-values