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.

lang/parser/parser.y

@@ -92,6 +92,7 @@ func init() {
 %token CLASS_IDENTIFIER INCLUDE_IDENTIFIER
 %token IMPORT_IDENTIFIER AS_IDENTIFIER
 %token COMMENT ERROR
+%token PANIC_IDENTIFIER
 
 // precedence table
 // "Operator precedence is determined by the line ordering of the declarations;
@@ -169,6 +170,11 @@ stmt:
 		posLast(yylex, yyDollar) // our pos
 		$$.stmt = $1.stmt
 	}
+|	panic
+	{
+		posLast(yylex, yyDollar) // our pos
+		$$.stmt = $1.stmt
+	}
 |	resource
 	{
 		posLast(yylex, yyDollar) // our pos
@@ -919,6 +925,22 @@ bind:
 		}
 	}
 ;
+panic:
+	// panic("some error")
+	PANIC_IDENTIFIER OPEN_PAREN call_args CLOSE_PAREN
+	{
+		posLast(yylex, yyDollar) // our pos
+		call := &ast.ExprCall{
+			Name: $1.str,
+			Args: $3.exprs,
+			//Var: false, // default
+		}
+		$$.stmt = &ast.StmtBind{
+			Ident: interfaces.PanicVarName, // make up a placeholder var
+			Value: call,
+		}
+	}
+;
 /* TODO: do we want to include this?
 // resource bind
 rbind: