Fix #281

• build MLX, Cmlx via xcodeproj as a framework
• expose C++ API at Cmlx framework level
• update mlx script handles file prep
• GitHub actions updated to build and test with these
• tool to build xcframework (binary distribution format)

Docs updated for use case:

Note that the SwiftPM and XCode (1) methods build MLX as a Library, not as a framework.
It is possible to construct a situation where YourApp -> MLX, YourApp -> YourFramework
and YourFramework -> MLX. This would give two copies of MLX in the resulting process
and it may not work as expected.

If this cannot be avoided, either by making YourFramework a Library or having YourApp
not link MLX, you can use the xcode/MLX.xcodeproj to build MLX as a Framework.
This will require mlx-swift to be checked out adjacent or inside your project,
possibly using git submodules, and dragging the mlx-swift/xcode/MLX.xcodeproj into
your project. Once that is done your application can build and link MLX and related
as Frameworks.

Ready to go!