lang: funcs: core: Add a second panic signature

This modifies the panic feature to accept a boolean or a string. If true
or not empty, then it will cause the panic. This makes some of the error
code a little less ugly.

examples/lang/panic/main.mcl

@@ -1,4 +1,10 @@
 class foo() {
+	panic(false) # should NOT panic
+	panic(false) # should NOT panic
+	panic("") # should NOT panic
+	panic("") # should NOT panic
+	panic(true) # should panic
+	panic(true) # should panic
 	panic("fail3") # should panic
 	panic("fail4") # should panic