Incremental compilation
nimsem: incremental structured cursor traversal
The change
nimsem walks the module structure with an incremental structured cursor rather than re-materializing the whole tree, cutting redundant work on re-check. This is groundwork toward incremental semchecking (#2064) — the machinery that lets a warm worker re-check only what changed.
Upstream: #2070.
Verification
Existing semcheck suite (byte-identical .s.nif output).