Use seed instead of rng in ranker

[btjanaka]

Description

Previously, we passed in an rng to every ranker method, which is inconsistent with other composed objects in pyribs like ES's. Thus, this PR switches rankers to take in a seed and manage their randomness by holding their own RNG.

TODO

Questions

Status

• [x] I have read the guidelines in CONTRIBUTING.md
• [x] I have formatted my code using yapf
• [x] I have tested my code by running pytest
• [x] I have linted my code with pylint
• [x] I have added a one-line description of my change to the changelog in HISTORY.md
• [x] This PR is ready to go