rust: Switch to 2021 edition

No real changes.

```
$ cargo fix --edition
note: Switching to Edition 2021 will enable the use of the version 2 feature resolver in Cargo.
This may cause some dependencies to be built with fewer features enabled than previously.
More information about the resolver changes may be found at https://doc.rust-lang.org/nightly/edition-guide/rust-2021/default-cargo-resolver.html
When building the following dependencies, the given features will no longer be used:

  libc v0.2.126 removed features: extra_traits

The following differences only apply when building with dev-dependencies:

  getrandom v0.2.6 removed features: std
```

which looks OK to me.

tests/inst/Cargo.toml

@@ -2,7 +2,7 @@
 name = "ostree-test"
 version = "0.1.0"
 authors = ["Colin Walters <walters@verbum.org>"]
-edition = "2018"
+edition = "2021"
 
 [workspace]