Rework spans source information and add more utility functions for spans

[xFrednet]

Span are a fundamental part of a linting interface. They define what should be shown in diagnostic messages and where to apply suggestions. Marker currently only has a limited Span implementation and a simple snippet function. Clippy and rustc both have a collection of util function to make it easier to deal with spans. It might be cool for Marker to have something similar.

See:

• rustc's SourceMap
• Clippy's clippy_utils::source

This util might need to be part of the marker_api crate to allow driver callbacks, similar to how the AstContext currently works :)

[xFrednet]

Spans in general should be updated to include more information about where they came from, like from which macro and if they have parent spans. See the fixme comment above the SpanSource struct.

[xFrednet]

I'll work on the source information and, in general, making spans more rounded. :)

Not sure if I'll complete this before v0.2.0, but we'll see :)

[xFrednet]

Just understanding all the variations of spans and macro expansion and documenting my current understanding is taking a significant amount of time. I never knew that spans were this complicated. In Clippy, we generally just abort if the code originates from a macro.

Oh well, when I have a working branch, this will probably be the best documented part of the API. Right now, it has about 150 lines of new doc comments. ^^

[xFrednet]

I've reworked the spans in #218, Marker should still include more util methods, but for now, I consider this issue to be resolved :)

I'll try create issues for more util methods soon :)