read/wasm: Expose data segments and linking SegmentInfo

[lapla-cogito]

Object::segments() previously returned nothing for wasm files because WasmSegment and WasmSegmentIterator were stubs, and the SegmentInfo subsection of the linking custom section was never parsed.

[philipc]

My expectation is that a relocatable object file wouldn't have any segments, and the name given here looks more like a section name to me. Maybe this should be a section instead?

[philipc]

I'm starting to realise that the Wasm support in this crate is a little flawed. The problem is that the Wasm file structure is quite different from other file formats, and doesn't fit cleanly in the unified API. We currently treat a Wasm section as the same thing as a section in other file formats, but I don't think we should. Semantically, a Wasm section is closer to a Mach-O load command or a PE data directory entry.

In particular, for data I don't think that a Wasm data section should be exposed in the unified API. It's merely a file format container for the data segments, and it is those data segments which are closer in meaning to an object::read::ObjectSection. For a linked Wasm file, those data segments should also be an ObjectSegment (similar to what we do for PE).

For the purposes of reading a relocatable object file, ObjectSegment should never be used. A linker should only be interested in the ObjectSections; it's the job of the linker to create the segments from the sections.

So if your goal with this PR is read the Wasm data segments from a relocatable object file, then I think a different change is needed. Exposing the Wasm data segments as ObjectSegment is still good, I'm just not sure that it is solving your problem.