make Key.t unboxed to save memory

[c-cube]

This should save 2 words per map entry.

[dbuenzli]

Sorry don't have this in my head. Is that orthogonal to #2 or does it solve it in a different manner ?

[c-cube]

I'm not exactly sure, I suspect it's orthogonal. I also have an implementation that stores more stuff in the key, I think. You can use as much code from there as you want.

This PR exists simply because I saw a simple fix to apply :-)

[dbuenzli]

I also have an implementation that stores more stuff in the key, I think

If you can afford (>= 4.08) you should use the technique here for your key UIDs, rather than a counter. Yes its Obj but its usage has been sanctioned by upstream when Type.Id was added to the stdlib

[c-cube]

Does this mean you can only have one key per type? I'm a bit confused...

[dbuenzli]

Does this mean you can only have one key per type?

No you have one type id per key. You can have many unequal type ids for the same type. See the docs here and the trivial sample hmap implementation that follows.