Remove C extension

[gdlg]

We are maintaining a C extension for rleEncode. This very small piece of code adds a build dependency to the C compiler (build-essentials) and sometimes causes troubles to users who choose to (or have to) install the library from source.

The goal is to remove this extension altogether, including any reference to it in the CI / build recipes, e.g. in pyproject.toml.

rleEncode can be either reimplemented in Rust (which we support anyway for other features), or we can use an off-the-shelf library (e.g. RLEMaskLib).