Add meta-definitions for AST types

- The big change here is that all the definitions for syntax-node classes have been macro-ized, to that we can do light metaprogramming over them
  - The use of macros for this has big down-sides, but I'm not quite ready to do anything more heavy-weight right now
  - The macro-ized definitions can be included multiple times, to generate different declarations/code as needed

- The first example of using this meta-programming facility is a new visitor system
  - The actual visitor base classes and the dispatch logic are all generated from the meta-files

- There was only one visitor left in the code: the semantics checker, so that was ported to the new system.

- All current test cases pass, so *of course* that means all is well.

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