Feature: Partial fallback

[conartist6]

At the moment this project has no partial fallback and no pretty-printing. When it enters fallback for a node all existing concrete syntax associated with that node is lost and it will print the absolute minimum amount of spaces that are necessary for compilation, and of course no comments.

I'm generally not too concerned with the pretty-printing part of it because I fully expect prettier to handle fixing up those parts. However for prettier to do its work I still need to ensure that I preserve the difference between '{ ' and '{\n' and any other space-sensitive patterns.

Comments are even more important to preserve than the pretty printing. We need to ensure that small changes like insertion into a list or removal from a list don't accidentally erase all comments attached to the list itself (which will likely be all of them).

[conartist6]

To get things working I had said up a NoSource but I think that will need to be eliminated, as we'll just have a TokensSource that is responsible for transitioning in and out of fallback.

[conartist6]

The biggest question is: how will a source transition out of fallback?

A correct solution needs to consider the reasons the source could be in fallback. The most common will be an insertion, deletion, or change to the node's tokens. This creates difficulty because I don't see any incremental way to recover. Recovery may involve passing over a token which is no longer relevant, or it may involve generating a token that did not exist, but there will be no way of knowing which of those things it is that needs to happen, and how many tokens the difference consists of.

[conartist6]

I also need to consider that items could be swapped

[conartist6]

OK at this point I'm just rehashing #3. I've been thinking about other things, but now that I'm back thinking about these problems I'm remembering all the reasons that I decided that pushdown was the correct solution.