I'm abandoning#1718 (at least for now). The attempt gave me a sense of how we might structure the codebase slightly differently in order to make it easier to add alternative code generators. Before getting to that point, I wanted to address validation — currently, it's a bit of a convoluted and wasteful process, which could be made much easier by moving validation into the node constructors, instead of having a separateValidator class which duplicates a lot of stuff.

The other notable changes so far in this PR are that the internalCompiler class becomesComponent (since it represents the component being compiled — it makes more sense to speak of e.g.component.transitions thancompiler.transitions). This means that the existingComponent class has been renamed toInlineComponent.

Also, thesrc/index.ts file has been radically simplified by moving things into the folders where they belong. My hope is that this will make it easier for people to ease their way into the codebase.