This module implements a simple database for mapping MIR SourceIds to non-critical meta-data (currently only a PNode).
Information about which PNode a MirNode originated from is needed for source-code position information and other reporting-related tasks.
The meta-data doesn't affect the semantics of MIR code and it's thus stored and implemented separately. This allows for using a different way of storing such information without requiring adjustments to the core MIR data structures.