engine: resources, lang: funcs, parser: Add panic magic

It's valuable to check your runtime values and to shut down the entire
engine in case something doesn't match. This patch adds some magic
plumbing to support a "panic" mechanism.

A new "panic" statement gets transparently converted into a panic
function and panic resource. The former errors if the input is not
empty. The latter must be present to consume the value, but doesn't
actually do anything.

examples/lang/panic/main.mcl

@@ -0,0 +1,8 @@
+class foo() {
+	panic("fail3") # should panic
+	panic("fail4") # should panic
+
+	test "nested-test" {
+		anotherstr => "hello!\n",
+	}
+}