Implement `leaf_index_to_mt_index_and_peak_index`

[einar-triton]

Note: This is works as intended, but as popcount becomes available shortly, this PR should be updated to use that before merge.

Add compile test Add run test

cargo test succeeds cargo clippy --all-targets succeeds

[Sword-Smith]

Since we are testing the compiler, we could have both versions of the algorithm: this and one using pop_count.

[einar-triton]

Alright. I will suffix this one with _loop and merge. Then _popcount can be added later.

[einar-triton]

wfm, merging