Make source of randomness configurable

[dhardy]

Currently Pippin has a dependence on the rand crate as a source of randomness used when finding element identifiers. This may not always be wanted: users may specify numbers while inserting new elements, or may prefer a different source of randomness.

Obvious solutions would be to make the source compile-time configurable (via a Cargo feature) or to get it from one of the user-provided traits.

• #0049