Update Cargo.toml with dependencies
Progress towards an MVP

Move and clean up tons of code, trying to get an application with the fully working flow of: `rustv install`, `rustv global 0.11.0`, `rustc --version`.
Fix installation paths not being properly set

The installation path was not properly being set to the versions directory. This commit also contains misc. clean up, and refactors.
Squash all warnings, and use IoResult over ()

Removed all warnings from the build output, and converted side-effecting code to use IoResult instead of implicit failure and ().