Current API for reading eFuse values is confusing/difficult to use

[jessebraham]

I've been meaning to open an issue for this for months now, but keep forgetting 😅

The current API for reading eFuse values is quite confusing, and requires doing math in order to port features over from esptool.py. We should try to better align the APIs to make it easier to adapt functionality and remove some potential sources of errors.

This functionality, IIRC, is purely internal, so we should be able to change it whenever without breaking the public API. However, I will add it to the v4 milestone regardless, as I would like to get it done :)