API, Rustc: Rework spans (and fix a few errors)

[xFrednet]

Spans are a fundamental part of linting in Rust. The previous implementation of spans in Marker were hacked together, with me always adding something new when it was needed. This PR reworks spans from the ground up.

The implementation is inspired by rustc's Span and connected data structures.

While testing, I also found a few ICEs and mistakes, that were fixed as well. The short summary:

• Marker's Spans are now cool
• Marker has less errors and panics
• Changes are almost net negative

An awesome effect of this change is that the Debug format implementation of spans looks a lot better now, like this:

path/file.rs:17:6 - 17:7,

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cc: #175

[Veetaha]

Nicely done 👍

[xFrednet]

Thank you for the review! ❤️