Allow for altExps with different features

[sjspielman]

As part of #150, we are enforcing that altExps of the same name (e.g., "adt") all have the same features during merging. In the future, we would like to allow for feature mismatch to merge altExps in cases where not all features will necessarily present in all altExps of that given name.

[sjspielman]

Something else to think about when the time comes, from #253 (comment):

I kept failures for different features, as I think that we may want to discuss what to do in the case of altExps with the same names but different feature sets: do we combine those or turn them into separately named altExps? I don't know yet, so failure seems the best option.

Edit: Noting there has been another vote for this approach.