We specialize `Optional<T>` for value types that inherently support some
kind of "empty" value or whose value range allow for a unlikely to be
useful sentinel value that can mean "empty", instead of the boolean flag
a regular Optional<T> needs to store. Because of padding, this often
means saving 4 to 8 bytes per instance.
By extending the new `SentinelOptional<T, Traits>`, these
specializations are significantly simplified to just having to define
what the sentinel value is, and how to identify a sentinel value.
This reverts commit c14173f651. We
should only annotate the minimum number of symbols that external
consumers actually use, so I am starting from scratch to do that
We can use the index's invalid state to signal an empty optional.
This makes Optional<StringTableIndex> 4 bytes instead of 8,
shrinking every bytecode instruction that uses these.
