Implement unstable Steenrod resolutions

[jfbaer]

It would be nice if the resolving could compute resolutions in the category of unstable modules over the Steenrod algebra.

[JoeyBF]

Yeah that's my bad, I should have made sure it wasn't already implemented before recommending to open an issue. For future reference, the relevant examples are resolve_unstable, unstable_chart and unstable_suspension