Add log1mexp and logdiffexp functions

[dylanhmorris]

Summary

Adds numerically stable implementations of log1mexp and logdiffexp, following Machler 2012. Provides a custom jvp for log1mexp to increase numerical stability near 0.

Closes #1368

Changes

• Adds functions and updates documentation to include them.
• Adds tests for the functions.

Testing / documentation.

• Verified test_distributions_util.py passes locally, including new tests.
• Confirmed documentation renders locally and that new docs render as expected.

Code cleanliness and style

pre-commit and linting / styling pass locally.

[fehiepsi]

Looks awesome! I think we can go ahead and use those utilities in several current distributions.