Add a builder for Agda packages.
Use agda builder for stdlib. mv to all-packages
Merge pull request #3915 from Fuuzetsu/agda-builder

RFC: Agda builder
nixos nat: add description to forwardPorts
libretro-cores:
  - adding: 4do, desmume, fceumm, mame, mupen64plus, scummvm,
            snes9x, stella, picodrive, ppsspp and vba

retroarch:
  - added retroarch wrapper for configured cores
  - changed attributes, old retroarch is now retroarchBare*