v1.12: recover `statementnumbers` functionality

[aviatesk]

For statementnumbers to function in v1.12, there's a need for a tool that can rebuild the full codelocs object from Core.DebugInfo, similar to what we could do just by looking at src.codelocs previously. Or, we may have to overhaul the statementnumbers algorithm.

Originally posted by @aviatesk in #606 (comment)