MLXArray (Data, Dtype) constructor , DType size.

[louen]

This PR provides a fix for #157 by offering a new convenience initializer for MLXArray.

MLXArray init from data

• new constructor for MLXArray taking Data, an optional shape (assuming a 1D array if unspecified) and a dtype.
• should behave similarly as existing constructor daking Data and an explicit HasDtype type.

Modifications to DType

• DType enum now conforms to Codable (saving the underlying raw value)
• DType enum is now CaseIterable (mostly useful for testing all dtypes)
• Dtype now expose size as a computed variable (Expose DType size through the C api mlx-c#39), the memory size of the associated type

[louen]

(rebased on latest main)

[davidkoski]

Thank you for the improvements!