Rsvm: install official Rust toolchains and switch them per shell

Hi everyone, I built rsvm: It downloads official standalone toolchains from static.rust-lang.org, verifies SHA256, and installs each one under ~/.rsvm/versions/. Several versions can be installed at the same time. Switching only changes PATH in the current shell. Multiple versions Each version is a full host toolchain in its own directory: ~/.rsvm/versions/1.85.0/ ~/.rsvm/versions/1.98.0/ ~/.rsvm/versions/1.100.0-nightly-2026-08-30/ rsvm install adds a version. It does not remove the othe...