Simplify ffi import/export

[Virgiel]

Which issue does this PR close?

Closes #4444

Rationale for this change

This simplification removes redundant code and simplifies the ffi logic around two functions :

pub fn to_ffi(data: &ArrayData) -> Result<(FFI_ArrowArray, FFI_ArrowSchema)> { }
pub fn from_ffi(array: FFI_ArrowArray, schema: &FFI_ArrowSchema) -> Result<ArrayData> {}

What changes are included in this PR?

The ArrowArray, ArrowArrayChild and ArrowArrayRef logic is hidden and simplified, but the internal logic remains the same.

Are there any user-facing changes?

ArrowArray is replaced with two functions : from_ffi and to_ffi.

[tustvold]

This makes sense to me, perhaps @viirya might be able to take a look as I believe you have more experience with this part of the codebase