Auto merge of #124545 - Zalathar:mappings, r=oli-obk

coverage: Split off `mappings.rs` from `spans.rs` and `from_mir.rs`

Originally, `spans.rs` was mainly concerned with extracting and post-processing spans from MIR, so that they could be used for block coverage instrumentation.

Over time it has organically expanded to include more responsibilities, especially relating to branch coverage and MC/DC coverage, that don't really fit its current name.

This PR therefore takes all the extra code that is *not* part of the old span-refinement engine, and moves it out into a new `mappings.rs` file.

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No functional changes. I have deliberately avoided doing any follow-up (such as renaming types or functions), because this particular change is very rot-prone, and I want it to be as simple and self-contained as possible.

`@rustbot` label +A-code-coverage

Author: bors