compiler: ensure we only visit nodes once

The way the compiler works right now is that a number of "transformers" will each visit the AST and run their own analysis.

And add-imports for example invokes a child visitor. I mean I've implemented that as a visitor on Program, but it's it really a visitor on Identifier? I guess the problem is that state needs to be passed to another process - althouhh perhaps it could just scribble somewhere

What we really need is to consolidate all the transformers into a single visit. This means that when traversing a huge file, we significantly reduce the CPU and memory overhead of compilation.

Some difficulties:

• what if one transformer wants to abort and ignore a whole subtree? Is this something we really need? If there's only one visit per node it doesn't feel so useful. Perhaps we can annotate paths and pass information downstream
• How do we control ordering? some transformers need to run before others right now (right??). That's harder to figure out in a single traversal