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.

lang/interpret_test/TestAstFunc2/panic0.txtar

@@ -0,0 +1,6 @@
+-- main.mcl --
+# This should not panic.
+panic("")
+panic("")
+-- OUTPUT --
+Vertex: _panic[panic]

lang/interpret_test/TestAstFunc2/panic1.txtar

@@ -0,0 +1,7 @@
+-- main.mcl --
+# This should not panic.
+panic("")
+test "hello" {}
+-- OUTPUT --
+Vertex: _panic[panic]
+Vertex: test[hello]

lang/interpret_test/TestAstFunc2/panic2.txtar

@@ -0,0 +1,6 @@
+-- main.mcl --
+# This should panic!
+panic("please panic")
+test "hello" {}
+-- OUTPUT --
+# err: errStream: func `panic @ 0x0000000000` stopped before it was loaded