add consistency checks in iterate

[mbauman]

• ensure that iterate generates length(I) values
• ensure that iterate always matches all of its skips against an index in the axis

addresses the most egregious bad behaviors in #7 and #31. They still don't work as you might hope, but now they throw errors rather than selecting a potentially wrong index.

[mbauman]

I think this is a good thing to do, whether we add in additional scalar to_indices calls (#41) or not.