rework buildsys to be able to use newer serde

[cgwalters]

I wanted to write a patch to start parsing JSON from Rust too, but to do that we can't use #[serde(deny_unknown_fields)]. I tried a few things, eventually landed at attempting:

#[derive(Serialize, Deserialize, Debug)]
#[serde(transparent)]
struct PermissiveTreeComposeConfig(TreeComposeConfig);

#[derive(Serialize, Deserialize, Debug)]
#[serde(transparent)]
#[serde(deny_unknown_fields)]
struct StrictTreeComposeConfig(TreeComposeConfig);

Except we can't actually use #[serde(transparent)] because of bindgen:

$ cargo update                                                                                                                                                                                                    
    Updating registry `https://github.com/rust-lang/crates.io-index`
error: failed to select a version for `serde_derive`.
    ... required by package `cbindgen v0.6.3`
    ... which is depended on by `rpmostree-rust v0.1.0 (file:///srv/walters/src/github/projectatomic/rpm-ostree/rust)`                                                                                            
versions that meet the requirements `= 1.0.21` are: 1.0.21

all possible versions conflict with previously selected packages.

  previously selected package `serde_derive v1.0.78`
    ... which is depended on by `rpmostree-rust v0.1.0 (file:///srv/walters/src/github/projectatomic/rpm-ostree/rust)` 

Which is the issue mentioned in this bugzilla.

So...ugh.

I guess what we need to do is split out bindgen into a separate src/bindgen build or something. The ugly bit will be teaching our vendoring bits about that.