Add recent_index, oldest_index, oldest, and is_filled to HistoryBuffer API

[YuhanLiin]

I added some convenience methods to the HistoryBuffer API to help keep track of both ends of the buffer.

[pleepleus]

I was just about to fork this repo because I needed these. I will be happy to review.

[pleepleus]

The code changes themselves look excellent. In fact I already pointed my project to this commit because I needed the "oldest()" api and it passes my application tests.

Before this can be merged though it should:

1. Refresh with a merge from main
2. Modify the changelog with a note on the additions

[newAM]

Sorry for the slow review! Life has been keeping me busy lately.

[newAM]

Thanks for making the change!

I took a closer look before merging, and I have one last question (sorry!). What is the use-case of the _index methods being public? That seems like an implementation detail.

[YuhanLiin]

The index methods can be used with as_slice (which also seems like an implementation detail) to do indexing operations relative to the head and tail indices.

[newAM]

Thanks for adding this! Sorry it took so long to get it merged.