build: add nix-shell and cargo at the root of SDK

This adds CI, formatting and clippy (linting) to the whole project's
rust code, so some fixes in the lib/ folder are necessary. The tests
still passes, nix-shell and nix-build works (with build outputting
the right expected format).

.cargo/config

@@ -0,0 +1,6 @@
+[target.'cfg(target_os = "linux")']
+# "-C link-args..." is required to get the system API calls exported
+rustflags = ["-W", "rust-2018-idioms", "-C", "link-args=-Wl,--export-dynamic"]
+
+[build]
+rustflags = ["-W", "rust-2018-idioms"]

.gitignore

@@ -4,6 +4,9 @@ result*
 build/
 target/
 
+# Nix-related directories.
+.cargo-home/
+
 # IDEs
 .idea/
 .vscode/