refactor: remove the rand runtime dependency and add common random bit function

[jeffoodchain]

Summary

Remove the rand runtime dependency from the published datasketches crate and add our own random generator.

KLL and REQ only require a random boolean when choosing which half of a
compaction range to promote. This PR replaces those calls with a small,
crate-private random_bit() implementation backed by a thread-local
SplitMix64 generator.

Closes #203

Changes

• add a crate-private, thread-local SplitMix64 random-bit generator
• seed each thread's generator on first use through RandomState
• use random_bit() for KLL and REQ compaction decisions
• remove rand from the kll and req feature dependency lists
• remove rand from the published library's normal dependencies
• document the dependency change in the changelog

The workspace still uses rand for benchmarks, but downstream users of the
datasketches crate no longer pull it in.

Compatibility

This does not change the public API or serialization format.

KLL and REQ compaction remain randomized. The new generator only replaces the
source of the random coin used to select which items are promoted.

Testing

• cargo x check
• cargo x lint
• cargo x test

[tisonkun]

Thanks!