When specifying `cbindgen` as a dependency via git, several 'skipping duplicate
package' warnings pop up regarding some of the test crates.
The warning seems to be spurious given that the test packages aren't needed when
depending on `cbindgen` (see https://github.com/rust-lang/cargo/issues/10752),
but while a fix is being considered for Cargo, this commit disambiguates the
duplicated package names by referring to their relative paths.
Opt-in since it uses a C++17 feature, but this allow exactly the same usage in
C++ and Rust, which I think is nice.
This also fixes constants of transparent structs in general, since the initial
version of this patch broke a test (associated constants in enums), so I added a
test for that too.
cbindgen currently assumes that the `Cargo.lock` is in directly in the
crate directory as a sibling to `Cargo.toml`; however that is usually
not the case inside a workspace.
Instead, this PR extracts the workspace root from the output of `cargo
metadata` [1] (already used to get a list of packages) and uses the
`Cargo.lock` from there.
1. This is available since Rust 1.24; see rust-lang/cargo#4940
