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[dlesbre]

Todo: check how this works with negative numbers

[dlesbre]

@mlemerre this should be ready to merge following your fix of int_builtins. Some points of note:

• We now support negative keys, as explained in QuickChecking Patricia Trees by Jan Mitgaard, this requires using an unsigned comparison instead of a signed one.
• I renamed the min_xxx/max_xxx functions to unsigned_min_xxx to avoid any accidental confusion, but it is an interface breaking change.
• I've switched the tests to using int as a generator instead of small_nat to test for problems with negatives/large numbers.

[mlemerre]

Except for the few commands, sounds good! I don't think the fix would introduces a noticeable slow down (but I am not sure the OCaml compiler can detect that this is just an unsigned comparison. Ideally we would have a primitive for this...). The names are not short but in the spirit of the library!