Decouple linux process specifics from elf dump file output

[dblnz]

Objective

I would like to use the ELF core dump generation on a system that does not have linux kernel.
The ELF core dump generation should work also on windows.
I need to create a core dump from the information I have about the guest running in a hypervisor isolated Hyperlight Sandbox (registers and guest memory).

Design decisions

• Maintain the existent public interface so that this does not introduce breaking changes
• Add the possibility to replace the Linux process as input for the ELF core dump creation
• Move the linux specific logic including the ProcessView type to a separate linux module that can be conditionally compiled when the platform is linux.
  This was not needed, but I thought it would be easier to follow.
• Make some of the internal types public so that they can be used by the custom process info source (implies modifying visibility and add required documentation)

Description

I defined a trait called ProcessInfoSource that would provide the necessary information the elfcore core dump output logic needs (pid, threads, regions, mapped_files, aux_vector, page_size).
The already defined ProcessView now implements this trait. The elfcore core dump output logic now takes a ProcessInfoSource trait object that provides all the needed information. This decouples the core dump generation from the ProcessView type that assumes a linux kernel.
To provide the custom process information source, the from_source method was added to CoreDumpBuilder to create a custom CoreDumpBuilder that takes the custom info from a provided source and memory reader.

[chris-oo]

The overall shape of the PR seems reasonable, but some minor nitpicks and questions about dead code.

[dblnz]

I took care of the comments above and modified the changes to address them and marked the conversation as resolved.
I modified the history of this branch to be easier to follow commit by commit.
I also added some tests and support for non linux platforms so we can generate ELF core dumps from ProcessInfoSource on any platforms.

[dblnz]

I added a commit that changes the crate internally from using trait objects to generics.
The consequence is we have some breaking changes:

• the CoreDumpBuilder struct expects two generic arguments
• the LinuxProcessMemoryReader (internal type that handles linux process memory reading) needs to be public now
• to create a core dump for a linux process one needs to use CoreDumpBuilder::<ProcessView, LinuxProcessMemoryReader>::new(pid) instead of CoreDumpBuilder::new(pid)

[chris-oo]

other than the nested imports, LGTM. Happy to merge once that's fixed.

I assume you'll need a new release on crates.io? I'll see about also getting some folks on my team to review.

[dblnz]

Thanks for the review.
A new release on crates.io would be great.