perf(coverage): use u32 for IDs, improve analysis

[DaniPopes]

• Use u32 for IDs and PC/IC to reduce memory usage
• Track "seen items" in find_anchors using source IDs instead of individual item IDs, as no items are shared between different sources. This drastically reduces hash lookups
• Merge items_by_source_id into SourceAnalysis to create the source ID -> coverage items in the analysis itself
• Simplify the above mapping from a HashMap<SourceId, Vec<ItemId>> to a Vec<(offset, length)>; this is possible because all source and item IDs are consecutive, see methods on SourceAnalysis and the find_anchors changes

The first commit is cleanup which is affected by the second one

This takes analysis down from 15s to 5s on superform-xyz/superform-core

[grandizzy]

makes sense, thank you!