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/parser/lexer.nex

@@ -231,6 +231,11 @@
 			}
 			return BOOL
 		}
+/panic/		{
+			yylex.pos(lval) // our pos
+			lval.str = yylex.Text()
+			return PANIC_IDENTIFIER
+		}
 /"(\\.|[^"])*"/
 		{	// This matches any number of the bracketed patterns
 			// that are surrounded by the two quotes on each side.