Start to redo evaluation using 0xfaded/go-interactive. For now there is slightly less functionality. However I think we will come back with a vengence.
gub/eval.go: EnvLookup needs to return an interp.Value not a string.
interp/value4gub.go: Use strconv.QuiteToASCII rather than a hacky warp in quotes, to get quoted escape sequences correct.