Draft: add VLCCore Rust API and first Rust modules

This is a draft MR, you don't have to review it yet. I will do a first review before.

This is the result of the work of

• Geoffroy Couprie for its initial VLC Rust port.
• Kartik Ohri during GSoC 2020
• Johan Pardo during an internship at Videolabs (under my supervision)

The following VLCCore API are ported to Rust

• Demux (with a flv example, using nom)
• Interface (with the listenbrainz example)
• Stream (with a cuesheet and inflate stream_filter examples)