High Level Direct Buffer Access API

[sjkelly]

In many readStream/writeStream calls, copies and conversions happen which can be suboptimal in many cases. Particularly in languages like Julia with multiple dispatch, the native resolution can be used until the latest possible point (or buffer overflow requires the handle to be freed).
An example of using the direct buffer access API is here: https://github.com/JuliaTelecom/SoapySDR.jl/blob/dff59201e244485e6db51e2454d08492df29a903/examples/buffer_access.jl