Rollup merge of #145392 - Zalathar:create-mappings, r=petrochenkov

coverage: Remove intermediate data structures from mapping creation

The data structures in `coverage::mappings` were historically very useful for isolating the details of mapping-extraction from the details of how coverage mappings are stored in MIR.

But because of various changes that have taken place over time, they now provide little value, and cause difficulty for the coordinated changes that will be needed for introducing expansion mapping support.

In the future, the pendulum might eventually swing back towards these being useful again, but we can always reintroduce suitable intermediate data structures if and when that happens. For now, the simplicity of not having this intermediate layer is a higher priority.

There should be no changes to compiler output.

Author: Zalathar