Add simple assert function to steal.dev object.

Closes #286.

@matthewp is this what you had in mine? Sorry about the diff, the indentation of the `dev` var was off and I couldn't help myself :fearful:; if this looks good to you I guess the next step would be to modify `steal-tools` to remove all the `steal.dev.assert` calls from the build. I'll check later how to do that. 

Thanks.