Generate .cargo/config

[scrogson]

This PR adds a .cargo/config to the generated skeleton to avoid issues
with running cargo build on macOS.

I've added a warning to the compiler task if compiling on macOS and the
.cargo/config does not exist.

Here is an example warning when the file doesn't exist:

λ mix compile
==> rustler
Compiling 1 file (.ex)
==> hyper
Compiling NIF crate :hyperbeam (native/hyperbeam)...

Compiling on macOS requires special link args in order to compile
correctly.

Rustler is currently working around this issue in the compiler task.
This will be removed in v1.0.0 in favor of a user supplied .cargo/config
file.

To remove this warning, please create /Users/scrogson/github/rusterlium/hyper/native/hyperbeam/.cargo/config
with the following content:

      [target.x86_64-apple-darwin]
      rustflags = [
          "-C", "link-arg=-undefined",
          "-C", "link-arg=dynamic_lookup",
      ]

See https://developer.apple.com/library/archive/documentation/DeveloperTools/Conceptual/MachOTopics/1-Articles/executing_files.html
for more details.


    Finished release [optimized] target(s) in 0.15s

Fixes #191

[evnu]

Does the config need to be replicated for all crates in a workspace, or is a single configuration per workspace enough?

[scrogson]

Does the config need to be replicated for all crates in a workspace, or is a single configuration per workspace enough?

@evnu I believe a single .cargo/config will work at the root of the workspace (as seen in the rustler repository itself).

[evnu]

I believe a single .cargo/config will work at the root of the workspace (as seen in the rustler repository itself).

Should the test here than traverse the directories to find any matching config?

[scrogson]

Should the test here than traverse the directories to find any matching config?

Not sure what you mean. It appears that a top-level .cargo/config would suffice.

Do you have a workspace-based NIF project you could test this hypothesis with?

[hansihe]

Looks good to me

[evnu]

Not sure what you mean. It appears that a top-level .cargo/config would suffice.

I think that the test for the configuration file here might output an "incorrect" warning, if the .cargo/config is not at the exact place where it is expected. This doesn't hurt, but might be confusing at first.