Improve Rust manifest API

[Tpt]

The kraken.hs.cargo.manifest module provides two datasources:

• The plain Cargo.toml file parsed into data structures (CargoManifest, Workspace, Package, Dependencies, Workspace, WorkspacePackage)
• The output of cargo metadata (CargoMetadata, WorkspaceMember, Artifact, Bin and Lib)

It presents multiple shotcomings imho:

• There is no way from a class name to know if it's from the plain Cargo.toml or from cargo metadata.
• The CargoMetadata datastructure looses information, for example the relation between WorkspaceMember and Artifact.

Proposal:

1. Create a new kraken.hs.cargo.metadata module presenting all data from cargo metadata version 1 and migrate all read-only usages from it.
2. Drop cargo metadata datastructures from kraken.hs.cargo.manifest
3. Simplify Cargo.toml representation (for example walking between the different crates of the same workspace might be done using cargo metadata)