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/interfaces/const.go

@@ -25,4 +25,10 @@ const (
 	// VarPrefix is the prefix character that precedes the variables
 	// identifier. For example, `$foo` or for a lambda, `$fn(42)`.
 	VarPrefix = "$"
+
+	// PanicResKind is the kind string used for the panic resource.
+	PanicResKind = "_panic"
+
+	// PanicVarName is the magic name used for the panic output var.
+	PanicVarName = "_panic"
 )