Low-level FFI bindings to imageflow

Example usage:

alias Imageflow.Native

# create a job
{:ok, job} = Native.create()

# add an input file, with id `0`
:ok = Native.add_input_file(job, 0, "input.jpg")

# you could also add input buffers directly from memory
:ok = Native.add_input_buffer(job, 1, <<0x89, 0x50, 0x4E, 0x47, 0x0D, ... >>)

# call `get_image_info` on buffer with id 0
{:ok, resp} = Native.message("v0.1/get_image_info", %{io_id: 0})