Rename `log2` to `log2_of_next_power_of_two`

Currently, there is a lot of ambiguity in the naming, requiring downstream user to check the original def to get confirmation of what it does. This makes it much clearer.

There is also precedent in the naming in usize methods, for the function `next_power_of_two`.

Search the repository for the `log2` definition and every call site; the issue provides no file or test path. Rename the function to `log2_of_next_power_of_two`, update references, and verify the Rust project's existing checks pass.

Written by the indexing model from the issue text.