Rust: ostree-rs v0.12.0 and ostree-rs-ext 0.2.1

["Colin Walters" <walters verbum org>]

ostree is currently a C library, as well as two Rust crates.
This is a release announcement for the Rust crates:

https://crates.io/crates/ostree/0.12.0
https://crates.io/crates/ostree-ext/0.2.1

Recently the glib crate released a new major version 0.14:
https://gtk-rs.org/blog/2020/07/04/new-release.html

As we're trying to add some new functionality in ostree-rs-ext (and rpm-ostree too e.g.), in particular 
OSTree's heavy use of GVariant wants good Rust-GVariant bindings, and the versions in glib 0.14 are *much* 
improved.  There's a lot of other changes there too of course.

This requires a major semver bump for us too, and while we're here some other cleanups happened.

# ostree-rs

The major change was the glib 0.14 bump, but we landed some other bits like:
https://github.com/ostreedev/ostree-rs/pull/6

# ostree-rs-ext

Also the glib 0.14 bump (and a bump to ostree v0.12).  As part of this we also dropped most of our custom 
`variant_utils` helpers.