debug safety for pointer casting to a struct or union

[andrewrk]

Normal Zig structs and unions do not guarantee size or field order. Zig is allowed to insert secret fields. So, for every normal struct that the code does a @ptrCast to or a @fieldParentPtr, insert a secret first field which has 4 bytes generated unique identifier for the type. When we do the cast, verify the id.

Debug safety that makes structs/unions bigger can be disabled with @setRuntimeSafety(false) in a comptime block directly inside a struct. Once #978 is implemented it would be @optimizeFor(.Small).

Of course, these protections are disabled by default in release-fast and release-small modes.