Add a justfile

`just` makes invocations of various tools more disprovable and gives a
quick way to run them.

justfile

@@ -0,0 +1,26 @@
+default:
+  @just --list
+
+# Cargo build everything.
+build:
+  cargo build --workspace --all-targets
+
+# Cargo check everything.
+check:
+  cargo check --workspace --all-targets
+
+# Lint everything.
+lint:
+  cargo clippy --workspace --all-targets -- --deny warnings
+
+# Run the formatter.
+fmt:
+  cargo fmt --all
+
+# Check the formatting.
+format:
+  cargo fmt --all --check
+
+# Test the workspace.
+test:
+  cargo test --workspace --all-targets