[WIP] Drop prototype features to focus on core functionality

[johnmyleswhite]

This removes several non-core pieces of functionality that we don't have the resources to maintain:

• Expression-based indexing
• The DataStream protocol
• The NamedArray type

Right now I haven't updated the tests to indicate the removal of functionality.

[nalimilan]

Removing NamedArray sounds like a good idea if we want to use this name for more general "named" or "associative" arrays.

Regarding with() and friends, why not move them to prototypes rather than removing them? I agree that's not a big difference, but I wonder whether you think they should be readded at some point, or whether a better alternative should be found.

[tshort]

I think the plan is to try out different approaches to with() and friends to see if we can overcome the problems with the existing implementations. This could be something like LINQ or a @select macro.

[johnmyleswhite]

I didn't move with to prototypes just to save programming effort. That's easy enough to undo. But I'm not totally sold on with (or any of the delayed evaluation tools) in the long term. It has two properties I strongly dislike:

• It duplicates functionality we already have. It's like syntactic sugar at the interface level, since you can always express a with operation by fully qualifying column names.
• It provides an implementation of core functionality that is non-trivially worse than the equivalent core functionality, because you need to understand subtle details about how Julia's expressions and scopes work to use with correctly. Saving a few characters doesn't seem to justify the very substantial conceptual complexity that with brings with it.

[nalimilan]

Yeah, with() is clearly syntactic sugar, but whether it is useful syntactic sugar or not depends on whether we find a better solution as @tshort said. I agree that for now it's OK to remove it.

[johnmyleswhite]

This is much closer to being done. The one trouble is that some of the code in formula.jl depends on with, so I need to fix that.

I changed the way tests are run to be more informative. If you type julia test/runtests.jl -f you'll get the old behavior where the first error is fatal. Otherwise you get a summary of each test's success/failure.

[johnmyleswhite]

I think this is ready for review now.

[johnmyleswhite]

I decided to not try to move with and friends to prototypes because I don't currently see a strategy for making them work well. For select, I opened another issue where I proposed using placeholders to get some of the effects of delayed evaluation. If people are ok with doing the same thing for with, I think we could make it work.

[johnmyleswhite]

Bump. I'd really like to merge this soon.

[tshort]

I like your deletions and reorganizations.

[johnmyleswhite]

I unfortunately can't merge this yet because the current formula definition depends on with. I'm working to fix it now.

[johnmyleswhite]

I made the minimum fix required to get simple GLM functionality working again. I'll do my best to restore everything else by the end of the night, but I'm going to merge this now to get it over with.