Module Option.Optional_syntax

You might think that it's pointless to have Optional_syntax on options because OCaml already has nice syntax for matching on options. The reason to have this here is that you might have, for example, a tuple of an option and some other type that supports Optional_syntax. Since Optional_syntax can only be opted into at the granularity of the whole match expression, we need this Optional_syntax support for options in order to use it for the other half of the tuple.