Universally quantified type variables

Implement universally quantified type variables, both explicitly given
by the user and inferred by the type inference algorithm.

tests/compile.rs

@@ -0,0 +1,41 @@
+use std::process::Command;
+
+use crate_root::root;
+
+const FIXTURES: &[(&str, i32)] = &[("simple", 5), ("functions", 9)];
+
+#[test]
+fn compile_and_run_files() {
+    let ach = root().unwrap().join("ach");
+
+    for (fixture, exit_code) in FIXTURES {
+        println!(">>> Testing: {}", fixture);
+
+        println!("    Running: `make {}`", fixture);
+        assert!(
+            Command::new("make")
+                .arg(fixture)
+                .current_dir(&ach)
+                .spawn()
+                .unwrap()
+                .wait()
+                .unwrap()
+                .success(),
+            "make failed"
+        );
+
+        let out_path = ach.join(fixture);
+        println!("    Running: `{}`", out_path.to_str().unwrap());
+        assert_eq!(
+            Command::new(out_path)
+                .spawn()
+                .unwrap()
+                .wait()
+                .unwrap()
+                .code()
+                .unwrap(),
+            *exit_code,
+        );
+        println!("    OK");
+    }
+}