Clean up different compiler versions setup

Currently updating to a new ghc version and adding the now-outdated version requires changes to every `iridium.yaml`. This should be simplified:
- The user should be able to globally define a list of ghc executables (paths)
- iridium should query their versions
- There should be a global default _version_ (not a path)
- Per-project, you should be able to either define a list of versions to be tested, one of which marked as default and used for commands that are run on only one compiler, or (if not defined) the default global default compiler will be used.

Start by inspecting the existing per-project iridium.yaml configuration and how GHC executables and versions are currently selected. Trace the configuration and command entry points before deciding where global and project-level settings belong. Done means compiler paths and versions no longer need to be repeated in every iridium.yaml, while global defaults and per-project tested/default versions work as described.

Written by the indexing model from the issue text.