refactor a bit

[chanced]

Hey @asmello, I made some changes for you to review. High level:

• renames SRC generic of Report to D and bound it to Diagnostic
• drops SUB in generic, uses D::Subject instead
• removes lifetime from Report
• removes seal for Diagnostic
• Adds associated type Related and method related to Diagnostic for multi-errors

Regarding lifetimes, I really think we should only deal with owned subjects. It simplifies the code considerably, reduces boilerplate, avoids borrowck issues, and doesn't cost much; a single allocation at worst on an error path for reporting seems more than fair. Given that this behavior is opt-in, I say we keep it simple.

I really don't know about the multi-error list. We can include it and I made some changes to support it, but I don't know how valuable it'll be. However, it is possible with this structure. To support it, we will need a struct ParseErrors(Box<[ParseError]> that would have a Related = ParseError and I'll need to move over the logic from the other branch.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 60.86957% with 9 lines in your changes missing coverage. Please review.

Project coverage is 96.0%. Comparing base (1404c61) to head (d844954).

Files with missing lines	Patch %	Lines
src/diagnostic.rs	57.1%	9 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/assign.rs	99.0% <ø> (ø)
src/pointer.rs	96.3% <100.0%> (-0.4%)	⬇️
src/diagnostic.rs	74.7% <57.1%> (-3.1%)	⬇️