Replace toml dependency with toml_edit to preserve comments

[danjl1100]

Motivation

As discussed in #394, crane currently uses the toml crate dependency in crane-resolve-workspace-inheritance.rs in order to merge the workspace root Cargo.toml with the current crate's Cargo.toml. This script is run on all git dependencies of the built crate.

One unfortunate side effect of using the toml crate is that all comments are removed from Cargo.toml. This is an issue if proc macros or build scripts are expecting to read non-TOML metadata from Cargo.toml (e.g. document-features used by some embedded libraries, such as embassy-stmf32)

Solution

This PR adapts the existing crane-resolve-workspace-inheritance.rs script to use toml_edit instead of the toml crate, to preserve comments.

Remarks

• There are two kinds of Tables in toml_edit, so some complexity (e.g. trait TableLikeExt) is needed to merge any combination of tables.
• The TOML key/value formatting cleanup is not strictly required, but I included this to keep the current smoke test mostly unchanged. One possible alternative is to use the toml crate as a dev-dependency, to only check the TOML is equal and avoid string equality; then the fmt module could be removed.

I tested this on my minimal example from #394, and it appears to work correctly. (reference danjl1100/example_uses_dep_using_document_features@7cc5174)

Checklist

• added tests to verify new behavior
• added an example template or updated an existing one
• updated docs/API.md (or general documentation) with changes
• updated CHANGELOG.md

[ipetkov]

Thanks for taking this on! Direction looks good, just have a few minor questions