resolve use of structure in structure definition

[zippy]

We currently do this to handle recursion and the semantics of doing so mean: "macro substitute" the definitions of the named structure in that place. I think this is the correct semantics, but we just need to make this explicit in the code somehow.

It will probably make _d_build_def_semtrex impossible in that it would introduce recursive semtrex. Which may mean that we have have to expand how semtrex works...