Consider removing `unsafe`

There are a few bits of code that use `unsafe` keyword. I wonder if we can rework it somehow to preserve performance but be fully "safe rust"? In the mean time, we should at least document why we think that code is safe.

Search the repository for each use of the Rust unsafe keyword and inspect the surrounding geometry and triangulation code. Determine whether each use can be replaced without losing performance; otherwise document why it is safe. Done means the unsafe sections are removed or their safety rationale is recorded.

Written by the indexing model from the issue text.