Feature to encode MLXArrays to .safetensors binary data in-memory

[grananqvist]

for data transfer purposes it would be very useful to be able to encode [String:MLXArray] into .safetensors format in the form of in-memory binary data type Data.

e.g. in addition to a FileWriter in mlx core https://github.com/ml-explore/mlx/blob/fd0d63ba5b83da41d6a0e75a0bcb5d70ec93eb40/mlx/io/load.h#L109, we can have a reader and writer to a binary data buffer, and expose this to swift apis, or something similar.

The purpose look like this:

weights: [String: MLXArray]
let blob: Data = save(weights) 
// ... blob can be transfered over the network
let sameWeights = load(blob)
// weights and sameWeights should be equal

[davidkoski]

Moving to mlx-c first -- this requires exposing a load/save function in io.h that uses an in memory byte buffer.