agda-stdlib: upgrade to 0.8 (they keep changing the version scheme)

Conflicts:
	pkgs/development/compilers/agda/stdlib-0.8.nix
	pkgs/top-level/haskell-packages.nix
agda: change defaults to latest version
Merge pull request #2992 from bluescreen303/agda

Agda: upgrade stdlib to 0.8, change defaults to latest version