ID ordering must do depth first not breadth first

[cirosantilli]

At cirodown 3254f4a , we can get surprising ID orders e.g.:

\M[1]

\q[\M[2]]

\M[3]

produces IDs that render on the document in order 1 3 2 rather than the desired 1 2 3.

This is because the \q is only searched for IDs after the toplevel \M[3].

Generally this is not a big problem because linkable things stay on toplevel, but still, should be fixed.

Maybe this will also completely break math macro definitions across math blocks: https://github.com/cirosantilli/cirodown/blob/73b52c4ce0e50945724a843ac12212faa2ded4bf/README.ciro#L870