Next release considerations

[caio]

Working on #12 I stumbled on several things I would like to change which would trigger api breakage (forcing a 2.0.0 release):

• Remove the centroid abstraction: it's useless in its current encantation, however it's exposed publicly
• Get rid of all explicit panic()s (the most annoying one is gone on avl-wip already)
• Configurable RNG (mostly to benefit testing)

Other things that would be nice to work on and don't necessarily force a major release:

• (Dropped from v2: doesn't seem worth it) More deterministic tests: currently running them with -run TestName yields different (but still correct of course) results when compared with running them all.
• Test different distributions: requires writing some code or adding test-only dependencies
• (Dropped from v2: blocked) Sync up merge formats after the reference implementation changes (issue87)
• Investigate prefix sum optimization ideas: using a binary indexed tree or a plain cache should improve speed noticeably without a big memory cost: d89c9f2

[caio]

I'll be updating branch v2 whenever I tick one of these boxes.

Permanent compare link: master...v2

[caio]

More notable changes going into this branch (dumping here so that it's easier for me to write the release):

• TDigest.Len() has been dropped: ef6ba11
• TDigest.Count() has been introduced (and it's not what Len() was): cb481be
• We can register more than 4B samples now: 4378773

[caio]

I've now added a CDF(float64) method: 7e8db71

Next time I sit down on this I'll release v2.0.0

[caio]

Released!