.arcconfig

rust-discovery: core implementation for take_quick_sample()

This makes in particular `rand` no longer a testing dependency.

We keep a seedable random generator on the `PartialDiscovery` object
itself, to avoid lengthy initialization.

In take_quick_sample() itself, we had to avoid keeping the reference
to `self.undecided` to cope with the mutable reference introduced
by the the call to `limit_sample`, but it's still manageable without
resorting to inner mutability.

Sampling being prone to be improved in the mid-term future, testing
is minimal, amounting to checking which code path got executed.

Differential Revision: https://phab.mercurial-scm.org/D6423

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