stage2: fix memory management of ZIR code

* free Module.Fn ZIR code when destroying the owner Decl
 * unreachable_safe and unreachable_unsafe are collapsed into one ZIR
   instruction with a safety flag.
 * astgen: emit an unreachable instruction for unreachable literals
 * don't forget to call deinit on ZIR code
 * astgen: implement some builtin functions