Audio effect/filter using Csound as audio engine

This plugin was originally developed in C quite a time ago: #681. I just came back and decided to get rid of the C implementation that I did at that moment, reviewed it and re-implement it in Rust. At this moment the plugin only contains a filter element, but later on, an src and sink elements could be added as well.

Motivation

As a Csound user, using GStreamer for interconnecting audio sources or sinks with all the available protocols and formats would be very helpful, and Gstreamer can be beneficiated by getting access to the Csound's audio processing/generators units called opcodes.

The development using the new Gstreamer bindings for Rust was very comfortable and funny. Hope to keep working with Rust and Gstreamer in my future projects. waiting for your feedback and suggestions.