Cargo: project system (and configuration)

[vosen]

Split off from #3.
Handle Cargo.toml file in the project system (allow for Cargo.toml being placed in non-root locations). Handle loading/saving/editing of Cargo.toml though project settings

[sherry-ummen]

Hello, wanted to know. Whether this will be done any time soon. Thanks

[vosen]

Hard to tell. I'm finishing whitespace-aware toml parsing here. Once it's done Il'll have to wrap it for C# and then start modifying Visual Rust to rely on it. The work progresses but I'm giving no hard guarantees or deadlines.

[Boddlnagg]

We should probably aim to use toml_edit for editing of Cargo.toml, since @vosen's toml_document is undocumented and hasn't seen progress in the last year. Furthermore it looks like toml_edit will be used by cargo-edit (see killercup/cargo-edit#69 (comment)), so it will be sufficiently battle-tested.

We will need a C# wrapper (through a C API) which partially exists for toml_document, but it should be possible to write that for toml_edit as well. The alternative would be a .NET implementation of a TOML library that preserves formatting, which is probably much harder.