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.

1 files changed, 1 insertions, 1 deletions

diff --git a/tests/diagnostics/while-predicate-type.slang.expected b/tests/diagnostics/while-predicate-type.slang.expected
index d921c588e..c65cb6ef8 100644
--- a/tests/diagnostics/while-predicate-type.slang.expected
+++ b/tests/diagnostics/while-predicate-type.slang.expected
@@ -1,6 +1,6 @@
 result code = -1
 standard error = {
-tests/diagnostics/while-predicate-type.slang(9): error 30010: 'while': expression must evaluate to int.
+tests/diagnostics/while-predicate-type.slang(9): error 30019: expected an expression of type 'bool', got 'S'
 }
 standard output = {
 }