feat: add foreign function support

tests/ffi_bindings.test

@@ -0,0 +1,17 @@
+[code]
+foreign fn putchar(c: i32) -> i32;
+
+fn main() -> i32 {
+    putchar(72);  // 'H'
+    putchar(105); // 'i'
+    putchar(33);  // '!'
+    putchar(10);  // '\n'
+
+    return 0;
+}
+
+[expected_return_code]
+0
+
+[expected_output]
+Hi!