Implement Nat.length_nat as an external primitive

[vouillon]

The current implementation of length_nat really depends on the representation of nats.
There is a hack to make it work with Js_of_ocaml, but it is not possible to do the same thing for Wasm_of_ocaml.
Implementing this function as an external primitive would resolve this issue and should not have much of a performance impact.

[xavierleroy]

(Note to Github Actions experts: the CI looks broken.)

Well spotted, thanks for the report. I was surprised that length_nat is defined in the C stubs but not used. I guess that's because the Obj-based implementation is faster when compiled with ocamlopt. But, as you say, it may not have much of a performance impact. Still, what about declaring length_nat as a "noalloc" primitive? This will remove some of the overhead.

(Note to self and other maintainers of Num: many other C primitives in Nat could be declared "noalloc".)

[dra27]

(ACK to the CI problem - it's being looked at)

[xavierleroy]

@vouillon : please add a "noalloc" attribute, and then I'll be happy to merge.

[vouillon]

@xavierleroy I have added the "noalloc" attribute.

[xavierleroy]

Thanks! Merged.