Removes the statistical crate from dependencies.

[wesc]

The StatisticalChunker implementation requires the statistical package for computing mean, median, and stddev. This PR implements those functions and removes the statistical crate. There are a few reasons to do this:

1. the statistical crate is ~10 years old and has several dependencies, one of which is rand_os. statistical is likely unmaintained (and in fact, it is almost impossible to Google search when looking for statistics libraries for Rust).
2. rand_os has a security advisory and has been merged into rand_core: https://rustsec.org/advisories/RUSTSEC-2025-0124
3. statistical pulls in a few dependencies that aren't necessary
4. In the wake of the recent high profile supply chain attacks, eliminating unmaintained dependencies is probably a good idea

test_statistical_chunker passes with the new implementations. Note that the old statistical implementations assert when input data doesn't hit a minimum length, and these new versions maintain the same behavior.

Here are the crates that are removed due to cutting out statistical:

• rand 0.6.5
• rand_chacha 0.1.1
• rand_hc 0.1.0
• rand_isaac 0.1.1
• rand_jitter 0.1.4
• rand_os 0.1.3
• rand_pcg 0.1.2
• rand_xorshift 0.1.1
• cloudabi 0.0.3
• autocfg 0.1.8
• num 0.2.1
• num-bigint 0.2.6
• num-complex 0.2.4
• num-rational 0.2.4

I generated this PR with the help of Claude Code, and I have reviewed every line of it.