Start 0.8 release cycle

After this commit, breaking changes will be merged until 0.8.0 is
tagged and pushed to crates.io.

Author: chrysn

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "riot-sys"
-version = "0.7.16"
+version = "0.8.0-alpha.0"
 authors = ["Christian Amsüss <chrysn@fsfe.org>"]
 edition = "2018"