Add some Shell.* wrappers around bitlash doCommand

Centralizing our interactions with bitlash, and making it easier to use Shell from a sketch.
It's not confusing at all, an empty string is a useless return, and if you want the string value (like for passing into evalArgs) it's terribly inconvenient to have to do this yourself.  A big point of adding all of this eval stuff is a convenience factor, and having this doesn't make anything impossible, only makes it more friendly.