s/,/:/ in what appears to be JSON.
Remove/refactor some .unwraps.

Many of these had some precondition being changed that guaranteed they'd
never happen, but could be expressed to just avoid the `.unwrap`
entirely (easier to check).

Other `Option` unwraps were changed to have a mildly more informative
error message by using `.expect`.
auto merge of #78 : huonw/cargo/minor-de-.unwrapping, r=alexcrichton

I don't think any of the `.unwrap`s I touched can actually be hit since they all seem to have checks earlier, but reducing the number of calls is always nice and makes the code easier to verify.

I'm not sure if the `,` -> `:` commit is actually correct, I was just working from the surrounding context.