feat!: `sourcemap` repr, str, docs and fixes

[antazoey]

What I did and How I Did It

My main goal here is trying to understand, so if you review this PR, please make help me do so!

So I noticed that __repr__ for SourceMap looked funny, so I starting poking in that. I found SourceMapItem and the parse() method so wanted to know more about how that worked because the docs were sort of missing.

• Adds __repr__ implementation to SourceMap
• Adds __str__ implementation to SourceMap
• Adds docs to SourceMapItem
• Renames stop to length in SourceMapItem

fixes: APE-581

How to verify it

Checklist

• [ ] Passes all linting checks (pre-commit and CI jobs)
• [ ] New test cases have been added and are passing
• [ ] Documentation has been updated
• [ ] PR title follows Conventional Commit standard (will be automatically included in the changelog)

[antazoey]

This diff will be better once this merges https://github.com/ApeWorX/ethpm-types/pull/59 ^ which contains all of the template updates