nexlint: introduce project linting

This introduces custom project linting using the nexlint linting
framework. As a start this patch enables the following lints via `cargo
xlint`:

* detection of duplicate versions of direct dependencies
* detection of irrelevant build dependencies
* checking of license headers on various file types
* ensuring the root Cargo.toml file is canonical

.cargo/config

@@ -8,3 +8,4 @@ xclippy = [
     "-Wclippy::all",
     "-Wclippy::disallowed_methods",
 ]
+xlint = "run --package x --bin x -- lint"

.github/workflows/rust.yml

@@ -44,7 +44,10 @@ jobs:
     runs-on: [ubuntu-latest]
     steps:
       - uses: actions/checkout@v3
+      - uses: actions-rs/toolchain@v1
+      - uses: bmwill/rust-cache@v1 # Fork of 'Swatinem/rust-cache' which allows caching additional paths
       - run: scripts/license_check.sh
+      - run: cargo xlint
 
   test:
     needs: diff