Rollup merge of #121019 - Zalathar:covspans, r=oli-obk

coverage: Simplify some parts of the coverage span refiner

This is another incremental step on my quest to dismantle the coverage span refiner into something more understandable and maintainable.

The biggest change here is splitting up `CoverageSpan` into several more specific structs. Doing so reveals that most of the places that were using that struct only need a subset of its fields and methods.

We can also get rid of separate tracking of `curr_original_span` and `prev_original_span`, by observing that `curr.span` never actually needs to be mutated, and that we can store `prev_original_span` directly in the dedicated struct for `prev`.

`@rustbot` label +A-code-coverage

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