ENH: Implement a function to find the longest decreasing streak

[seberg]

We would like to have a small function that finds the longest streak of decreasing values.
For example given the list:

• [3, 1, 2, 3, 2, 4] would find [1, 2, 3] at index 1.
• [1, 2, 0, 10, 100] would find [0, 100, 100].

Working on this issue

Implementing bubblesort consists of the following things:

• Find the existing longest_decreasing_streak stub and replace it with your implementation.
• Add documentation (you can look at largest_lement or merge_sort for inspiration).
• Add some tests by expanding the test_longest_decreasing_streak in test_inspect.py.
• Once you have made your changes, and committed them to a new branch, you can create a PR!

(This is similar to gh-17, but we can work on two different implementations in parallel.)