Bump IndexMap to V2, resolve some Clippy lint suggestions

[martingallagher]

No description provided.

[ahl]

it's disappointing that indexmap renamed the feature; otherwise we could just depend on a range of versions. As it is, I don't see how to update the indexmap dependency without it being a breaking change (i.e. moving this crate to 2.0). Any thoughts?

[martingallagher]

Maybe a re-export of indexmap, like the datafusion suite of crates? Datafusion re-exports sqlparser for example (its local dep is often out of sync with the latest sqlparser).

You're right about the pain involved, this PR is a consequence of trying to update indexmap on a project, I didn't fully consider the consequences. Perhaps this PR can be ignored and users can import a indexmap_v1 package alias, but going forward maybe the re-export pattern could be useful, I doubt I'm alone in wanting to update the indexmap version in my projects.

[ahl]

100% with you -- I want to update indexmap elsewhere as well. Just trying to navigate past this in the least painful way. I wish that indexmap had retained the serde-1 feature for this case...

[ahl]

I found a way to update the dependency in a non-breaking way: #76. Feel free to update this PR with the clippy changes if you'd like. Please let me know if my approach worked (or didn't work) for you.