Currently, stage1 runs astgen for every comptime function call,
resulting in identifier strings being allocated multiple times,
wasting memory. As a workaround until the code is adjusted to
make astgen run only once per source node, we memoize the
result into the AST.

 * Rename `ir_gen_*` to `astgen_*`
   - Oops, meant to do this in a separate commit. My bad.
 * tokenizer: avoid using designated initializer syntax.
   MSVC does not support it.