Bump version

Changes:

* Use C2Rust in addition to bindgen
* Enable more headers, eg. for CoAP, sockets and all peripherals
* Enable building for docs.rs by having a slimmed-down copy of the
  headers checked in (not fully active yet as it depends on [291] and a
  release from there)

[291]: immunant/c2rust#291

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "riot-sys"
-version = "0.2.4"
+version = "0.3.0"
 authors = ["Christian M. Amsüss <ca@etonomy.org>"]
 edition = "2018"